Detecting computer security risk based on previously observed communications

ABSTRACT

Information about an electronic message that is from a sender for an intended recipient is received. It is determined whether an electronic message account of the sender of the electronic message is likely an independently controlled account. In response to the determination that the electronic message account of the sender of the electronic message is likely an independently controlled account, the electronic message is analyzed to determine whether the message is an automatically generated message. In response to the determination that the message is an automatically generated message, a security action is performed.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/428,328 entitled ADDRESSING ACCOUNT-TAKEOVERS filed Nov. 30, 2016, and claims priority to U.S. Provisional Patent Application No. 62/490,309 entitled DETECTION OF ABUSE INVOLVING ASSOCIATED ACCOUNTS filed Apr. 26, 2017, both of which are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

There are many ways in which computer hackers and scammers deceive potential victims. Many of these ways involve assuming an identity of a party the potential victim trusts. This may be an individual, such as a friend or a colleague, or it may be a brand, whether one that the potential victim has a relationship with or simply knows of as trustworthy.

A common technique used by computer hackers is to compromise an electronic message account, e.g., using phishing or malware that gains access to the account, and then search the mail folders of the victim for emails that indicate connections and content of potential value in the context of an attack. Upon finding interesting information, such as information indicating that a second party has a trust relationship with the user owning the compromised account, the hacker commonly uses the compromised account to send an email to a second party. This is done with the hope that the second party will be willing to respond in the intended manner to the email, given that it comes from a party he or she trusts.

Sometimes hackers also use compromised accounts to send messages to parties that do not have a trust relationship with the owners of the compromised accounts. This is to circumvent reputation-based filtering that filters emails that come from accounts without a positive reputation (e.g., newly created accounts) and accounts with a poor reputation (e.g., accounts that have been used extensively to send unwanted emails.)

A compromised account is commonly referred to as an account that has been taken over. The action of compromising is commonly referred to as an account take-over (ATO). Compromised accounts are also referred to as ATOed accounts. There is a need for ways to more effectively and efficiently detect and mitigate effects of ATOed accounts.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.

FIG. 1 is a diagram illustrating an embodiment of a system environment for analyzing a message.

FIG. 2 is a flowchart illustrating an embodiment of a process for analyzing a received message for security risks.

FIG. 3 is a flowchart illustrating an embodiment of a process for analyzing individual aspects of a message for security threats.

FIGS. 4A-4F show various examples of headers included in example email messages.

FIG. 5 is a flowchart illustrating an embodiment of a process for analyzing aspects of a received message based on whether the message is from an independently controlled account of a sender.

FIG. 6 is a flowchart illustrating an embodiment of a process for wrapping encrypted content of the message.

FIG. 7 is a flowchart illustrating an embodiment of a process for tracking trusted contacts.

FIG. 8 is a flowchart illustrating an embodiment of a process for determining information about a message sender to be utilized to assess a risk of a message.

FIG. 9 is a flowchart illustrating an embodiment of a process for determining a total risk measure of a contact.

FIG. 10 is a flowchart illustrating an embodiment of a process for assessing a received message for filtering.

FIG. 11A is a flowchart illustrating an embodiment of a process for modifying a display name of a sender of the message.

FIG. 11B is a flowchart illustrating an embodiment of a process for modifying a reply address of a sender of the message.

FIG. 12 is a flowchart illustrating an embodiment of a process for securing a message.

FIG. 13A is a flowchart illustrating an embodiment of a process for detecting misrecognition of a sender of a message by a recipient of the message.

FIG. 13B shows an example of a modified message requesting manual classification of the sender of the message.

FIG. 14A is a flowchart illustrating an embodiment of a process for requiring a sender of a message to verify an identity of the sender.

FIG. 14B shows an example of an automatic reply message requesting verification of an identity of the sender of the message.

FIG. 15 is a flowchart illustrating an embodiment of a process for determining a change in historical reputation of a sender of a message.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as a process; an apparatus; a system; a composition of matter; a computer program product embodied on a computer readable storage medium; and/or a processor, such as a processor configured to execute instructions stored on and/or provided by a memory coupled to the processor. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. Unless stated otherwise, a component such as a processor or a memory described as being configured to perform a task may be implemented as a general component that is temporarily configured to perform the task at a given time or a specific component that is manufactured to perform the task. As used herein, the term ‘processor’ refers to one or more devices, circuits, and/or processing cores configured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.

In some embodiments, a received electronic message is analyzed to identify signs indicative of a message coming from an ATOed message account. If the message is identified as being of a sufficient high risk of coming from an account that has been potentially ATOed, a security action is performed in an attempt to minimize a security risk associated with the message. It is desirable to reduce the false positive and false negatives such ATOed account detection.

In some embodiments, it is determined whether a sender of an electronic message has an established relationship with an intended recipient of the electronic message. For example, if the sender of the message has previously communicated with the intended recipient a sufficient number of times for at least a threshold length of time, the sender of the electronic message is determined to have an established relationship with the intended recipient. In response to a determination that the sender of the electronic message has an established relationship with the intended recipient, the electronic message is analyzed based on at least in part previously observed communication between the sender and the intended recipient to determine a security risk of the electronic message for the intended recipient. For example, given what is already known about the sender of the message based on a classification and analysis of previous communications between the sender and the intended recipient, it can be determined whether the newly received message conforms to a pattern detected in the previous communications. Based on the result of the analysis of the electronic message, a security action is performed, if applicable. For example, the electronic message may be blocked if a sufficiently high level of risk is detected and/or the message may be modified to include a warning about the ATOed account risk if a sufficient medium level of risk is detected based on the analysis. If no or low level of risk is detected, the message may be allowed to be access by the intended recipient by delivering the message to a message inbox of the recipient.

In some embodiments, in response to a determination that the sender of the electronic message does not have an established relationship with the intended recipient, it is determined whether an electronic message account of the sender of the electronic message is likely an independently controlled account. For example, a message that was sent from an account that belongs to a large organization that closely controls who can send a message via its domain is not an independently controlled account whereas a personal email message account is an independently controlled account. In response to a determination that the electronic message account of the sender of the electronic message is likely an independently controlled account, the message is analyzed to determine whether the message is an automatically generated message. For example, a header of the message is analyzed to determine whether the message was automatically generated using a script or a program. In response to a determination that the message is an automatically generated message, a security action is performed. For example, the electronic message may be blocked if a sufficiently high level of risk is detected and/or the message may be modified to include a warning about a security risk if a sufficiently medium level of risk is detected based on the analysis. If no or low level of risk is detected, the message may be allowed to be access by the intended recipient by delivering the message to a message inbox of the recipient.

FIG. 1 is a diagram illustrating an embodiment of a system environment for analyzing a message. Examples of the message include an electronic mail (i.e., email), an instant message, a text message, a Short Message Service (SMS) message, a text message, a Multimedia Messaging Service (MMS) message, and any other forms of electronic messages. Analysis server 102 is connected to recipient message server 106 via network 108. In some embodiments, analysis server 102 is directly embedded or implemented in recipient message server 106. Sending message server 104 sends a message to recipient message server 106 for delivery to a recipient associated with recipient message server 106. In some embodiments, recipient message server 106 is a local message server. Gateway 110 receives the message prior to delivery to recipient message server 106. Gateway 110 may process incoming email messages for one or more recipient users of an organization (e.g., for users with a common domain email address). In various embodiments, recipient message server 106 may be any type of server that receives a message for delivery to a recipient user. Sending message server 104 is the last server that handled a message prior to delivery to recipient message server 106 via network 108. For example, sending message server 104 is an outgoing email server of a sender of the message. In various embodiments, sending message server 104 may be any type of server able to send a message to recipient message server 106.

Analysis server 102 processes a received message and filters it for potentially harmful or undesired messages. For example, incoming messages are filtered and analyzed for spam, viruses, spoofing, impersonation, ATOed sender, and any other harmful or undesired content to detect, prevent, or minimize such messages from reaching a message recipient served by the recipient message server. In some embodiments, analysis server 102 determines and/or maintains a list of trusted contacts for one or more users (e.g., user accounts) of recipient message server 106. For example, by obtaining an address book from a user and/or observing message traffic to and from server 106 of the user, server 102 determines and/or maintains a list for each user.

In some embodiments, a message sent from sending message server 104 is first received at analysis server 102 prior to being received at gateway 110 and recipient message server 106. In some embodiments, a message sent from sending message server 104 is first received at gateway 110 that sends the message to analysis server 102. In some embodiments, a message sent from sending message server 104 is received at gateway 110 that sends the message to recipient message server 106 and recipient message server 106 send the message to analysis server 102. In an alternative embodiment, analysis server 102 is included in gateway 110. In an alternative embodiment, analysis server 102 is included in message server 106.

In addition to analyzing the message, analysis server 102 may block and/or modify the message or instruct another server (e.g., instruct server 106) to block and/or modify the message in the event a potential threat is detected. In some embodiments, analysis server 102 initiates a user inquiry regarding a sender of the message in the event a potential threat is detected. In some embodiments, analysis server 102 receives information about one or more messages sent by a user of message server 106 (e.g., receives the message or a portion of the message, a recipient identifier included in the message, etc.). This information may be utilized by analysis server 102 to identify message behavior and/or message contacts of the user.

In some embodiments, recipient message server 106 performs a risk analysis for an incoming message at least in part by performing an authenticity and/or reputation analysis to determine an overall measure of risk (e.g., risk score). Performing authenticity analysis may include determining a measure of confidence that a sender identified in the message (e.g., domain of sender) is the actual sender of the message. Performing reputation analysis may include determining a measure that an identified sender of the email (e.g., domain of sender) is likely to send a message that is of value to a recipient (e.g., likelihood of sending message that a recipient would want/desire to receive).

In some embodiments, a risk analysis performed by recipient message server 106 results in a risk value of a message. The risk value may be utilized at least in part to flag/label the message, modify the message (e.g., add warning), and/or to determine whether to allow the message to be delivered to an intended recipient of the message.

Examples of network 108 include one or more of the following: a direct or indirect physical communication connection, a mobile communication network, Internet, intranet, Local Area Network, Wide Area Network, Storage Area Network, and any other form of connecting two or more systems, components, or storage devices together. Other communication paths may exist and the example of FIG. 1 has been simplified to illustrate the example clearly. Although single instances of many of the components shown in FIG. 1 have been shown to simplify the diagram, additional instances of any of the components shown in FIG. 1 may exist. For example, a plurality of recipient message servers are connected and/or managed by analysis server 102. Multiple analysis servers may exist. Multiple recipient message servers may serve the same recipient entity/domain. Components not shown in FIG. 1 may also exist. Any of the components shown in FIG. 1 may be a logical, physical, or virtual component.

FIG. 2 is a flowchart illustrating an embodiment of a process for analyzing a received message for security risks. For example, the process of FIG. 2 is utilized to detect account take-over risk of a sender of an electronic message and take responsive action to mitigate potential effects of the risk. The process of FIG. 2 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1.

At 202, information about a received message (e.g., message to be delivered to an intended recipient) is received. In some embodiments, the information is received at analysis server 102 of FIG. 1. The information may include information included in or related to the message. For example, the information includes email address header information including an email address and a display name of the sender of the message. In some embodiments, the information about the message includes contents of the message (e.g., a copy of the message). Examples of the message include an electronic mail (i.e., email), an instant message, a chat message, and any other forms of electronic messages. The information may be received as a part of a computer security analysis and filtering to identify and neutralize security threats prior to delivering the message and allowing an intended recipient of the message to access the message. In some embodiments, each message received for delivery to its intended recipient associated with the system performing analysis of the message is to be analyzed and information about each of the messages is received for analysis.

At 204, the message is profiled. In some embodiments, step 204 is only performed if it is determined that a security risk associated the message is below a threshold (e.g., risk score determined in 208 and/or 210 is below a threshold). Profiling the message includes storing information about and/or included in the message in a database to track historical observations about the sender of the message. For example, in identifying accounts that might have been ATOed, messages sent by the account are to be analyzed based on past observations about typical messages sent by the sender and the degree to which a newly received message from the sender deviates from what has been typically observed about messages sent by the account of the sender. In some embodiments, the historical information about messages by the sender for a particular recipient may be combined with historical information about messages sent by the sender for other recipients to determine a combined message profile/history of the sender for use in assessing security risk of messages by the sender for recipient without an established communication history with the sender. In some embodiments, profiling the message includes profiling, tracking and/or storing content and/or header information of the message in a data structure that tracks historical information about messages sent by the sender for a particular recipient. In some embodiments, profiling the message includes profiling, tracking and/or storing content included in the message in a data structure that tracks historical information about types of content typically included in messages sent by the sender for a particular recipient. For example, the data structure tracks keywords and/or content topic/type included in messages sent by the sender for a particular message recipient and keywords and/or content topic/type detected in the newly received message are used to update this data structure.

In some embodiments, at least a portion of header information of the message is stored in profiling the message. In some embodiments, select information included in the header of the message is tracked and associated with a history message profile of the sender. For example, information in the header of the message that identifies one or more of the following is tracked for the sender of the message: a mail user agent, a time zone, IP address, X-headers, supported character sets, and any desired information of interest in the header. One or more different profiles for the sender of the message may be updated using the message. For example, certain header information and/or content of messages of the sender may be correlated with each other and associated groups of header entries and/or content entries of messages of the sender are updated when profiling the message. In one example, different profiles of the sender are categorized based on different devices used by the sender to send messages to a particular recipient.

The table below shows different header information that is collected to build different profiles about messages sent using different devices of the sender.

Android Feature iphone Mac browser iPad phone DKIM yes yes no no yes signature? Mime 1.0 (1.0) 1.0 (Mac OS 1.0 1.0 (1.0) 1.0 version X Mail 10.1\ (3251\)) Content type text/plain; text/plain; text/html; text/plain; text/plain; charset=us- charset=us- charset=UTF- charset=us- charset=UTF- ascii ascii 8 ascii 8 X-Mailer iPhone Mail Apple Mail N/A iPad Mail N/A (14B72) (2.3251) (14B100) Message ID Type 1 Type 1 Type 2 (both Type 1 Type 2 (both formatting formatting and and subdomain) subdomain) Cipher TLS1_2 TLS1_2 N/A TLS1_2 TLS1_2 This table shows that by recording the header entries historically associated with messages from the same sender, a prediction can be made in terms of whether a newly received message and aspects of the message in combination are consistent with the past observations. Each trusted sender that has sent a message to a specific recipient is associated with a record that describes what different header configurations have been observed for the sender. This can be done both using a time window approach that emphasizes recently observed headers over very old headers, or an approach that takes into account all saved headers. Along with headers or portions of these, the data structure can store counts (e.g., how many times these were observed) and time intervals for the observations. For example, the number of times a particular message feature (e.g., message header item) was observed in received messages from the sender within a recent window of time (e.g., within a threshold time period and/or numerical count of past messages) and timestamps of each associated received message from the sender can be tracked and stored.

In some embodiments, statistics on the commonality of combinations of tracked aspects of messages (e.g., header entry, message body content, etc.) of a sender are determined and updated based on the newly received message. These statistics allow a determination of the likelihood that various aspects of a message would be observed together in the message. This also can be used to determine the likelihood of a new aspect detect in a message from the sender conforms to previously detected changes over time in previous messages from the sender. For example, this information can be used to determine the likelihood that the sender would change from one computing platform to another, the probability that the sender would upgrade their system, and the probability that the sender would use another machine temporarily. Thus message profile data of previously received messages from the sender can be used to determine a Bayesian probability score of a particular message feature being consistent with past observations for a communication between the sender and the recipient, given the context of other aspects of the message and a pattern of previous changes as a function of time.

In some embodiments, profiling the message includes identifying content included in a message signature and updating a data structure that tracks observations about message signatures of messages from the sender and associated context in which particular message signatures were utilized. For example, if a message signature that states “Sent from my iPhone” has been tracked and observed in a sufficient number of messages sent by the sender using a mobile phone message client and if a newly received message includes this signature but was sent using a desktop computer message client, this abnormality can be detected based on the historical observations about message signatures of the sender.

At 206, it is determined whether the message was received from a sender that has an established relationship with an intended recipient of the message. In some embodiments, the sender of the message has an established relationship with the intended recipient of the message if the sender (e.g., from email address of the sender) has sent a message to the intended recipient of the message (e.g., recipient email address of the message) a threshold number of times, if the intended recipient has previously sent a message to the sender a threshold number of times and/or if the sender has been sending messages to the intended recipient of the message for a threshold length of time. For example, the sender of the message is the established relationship contact of the intended recipient of the message if a threshold number of messages have been exchanged between the sender and the recipient. In some embodiments, a sender model developed for the sender of the message is utilized to determine whether the sender is an established relationship contact of an intended recipient of the message.

In some embodiments, determining whether the message was received from an established relationship contact of an intended recipient of the message includes determining a trust score associated with the sender of the message for the intended recipient of the message. If the trust score is above a threshold, the sender is identified as having an established relationship with the recipient and otherwise, the sender is identified as not having an established relationship with the recipient. The trust score indicates the closeness of the relationship between the sender and the intended recipient. This can either be a binary predicate or a real number. In a simplified example, the trust score can be set to 1 (e.g., indicating established relationship) if each of the two parties has sent the other at least three messages over a course of no shorter than one month, and otherwise to 0 (e.g., indicating not an established relationship). In another example, the trust score can be a function of the number of messages received by the recipient organization sent by the sender's organization and the number of messages that have been identified as undesirable, whether by the classifier or by a recipient.

In some embodiments, the trust score of the message identifies the extent to which the system recognizes the sender based on its historical message traffic; as such, it may not be limited to the history of the recipient or her organization. For example, a message from a sender with a very brief history would receive a low trust score, especially if the communication pattern is similar to that of previously identified malicious senders. A sender with a longer history and low traffic volume (but very few or no complaints of past abuse) would have a higher trust score, and a sender with a long history, high volume, and few or no complaints would correspond to an even higher trust score.

In some embodiments, determining whether the sender of the message has an established relationship with the intended recipient includes determining whether the sender of the message is included in a contact list (e.g., address) of the intended recipient. For example, if the sender of message is included in the contact list of the intended recipient and a sufficient number of the message was previously exchanged between the sender and the recipient, it is determined that the sender has an established relationship with the recipient.

In some embodiments, determining whether the sender of the message has an established relationship with the intended recipient includes obtaining a social, organizational, communication, collaboration, business and/or other relationship information of the sender and/or the intended recipient. Information about the sender and/or other users connected/related to the intended recipient may be obtained by requesting the desired information from a service (e.g., an external third-party service accessed via an Application Programming Interface such as Microsoft Graph API). For example, an identifier of the intended recipient of the message is provided to the service, and the service provides a list of other users connected to the intended recipient (e.g., connected via a social network, organizational relationship, business contact, etc.) that is ordered by the various other user's relevance to the intended recipient (e.g., order based on previous communication, collaboration, business relationships, etc.). It is determined that the sender of the message has an established relationship with the intended recipient if the sender is included in this list as having a sufficient relationship with the intended recipient (e.g., included in the list). Otherwise it is determined that the sender of the message does not have an established relationship with the intended recipient. In another example, an identifier (e.g., email address) of the sender of the message is provided to the service and the service provides information about the sender (e.g., information about the sender gathered across various different message repositories, contact lists and social networks). This information about the sender is used to assess a strength of a relationship between the sender and the intended recipient of the message (e.g., along with message history between them), and if the strength value of the relationship is greater than a threshold value, it is determined that the sender of the message has an established relationship with the intended recipient. Otherwise it is determined that the sender of the message does not have an established relationship with the intended recipient.

If at 206, it is determined that the message was received from the sender that has an established relationship contact with the intended recipient of the message, at 208 the message is analyzed for security risks using historical observations associated with the sender of the message with respect to the intended recipient of the message.

An example of the historical observations is the historical observations updated in 204. The historical observations include information about messages from/to the sender of the message to/from the recipient of the received message. In some embodiments, there exists a plurality of aspects of the message to be analyzed using the historical observations, and each aspect is analyzed to determine a risk component score associated with each component. These component scores are then combined (e.g., added, weighted then added, averaged, etc.) to determine an overall risk score. In some embodiments, each of the component scores is associated with one or more specific types of risk and a separate total score is calculated for each of the different types of risk based on its associated component scores. For example, there exists ten risk component scores and five of the component scores are only associated with a malware risk type, four of the component scores are only associated with a phishing risk type, and one of the component scores is associated with both the malware risk type and the phishing risk type. In this example, a total malware risk type score is calculated using the six of the ten risk component scores that are associated with the malware risk type while the total phishing risk type score is calculated using the five of the ten risk component scores that are associated with the phishing risk type. Examples of the aspects of the message analyzed using historical observations include message content keyword, a mail user agent, a time zone, an IP address, an X-header, supported character sets, a message signature, and any information about the message.

In some embodiments, using past observation data generated by profiling past messages from the sender to the specific recipient of the message, various data and statistics as well as clusters of metadata/configurations/content of messages that are likely to be observed together in combination in a message from the sender can be determined. For example, using past observation data generated by profiling past messages from the sender, statistics on the commonality of various combinations of metadata/configurations/content of messages have been determined and stored in a data structure. Using this information in combination with identified information in the contents and/or header of the message, a likelihood the received message conforms to a typical message from the sender for the intended recipient is determined. This likelihood takes into account statistics of likely changes over time. For example, the determined risk reflects the likelihood that the sender would change from one platform to another, the probability that the sender would upgrade their computer system, and the probability that the sender would use a new machine temporarily. Thus, a component risk score value for an aspect of the message may be computed using a Bayesian probability score of the particular observation/feature/content/metadata being consistent with past observations of communications between the sender and the recipient, given the context of other aspects of the message and a pattern of previous changes as a function of time. In some embodiments, past observation data about messages from the sender is analyzed to determine a probability that a combination of features/contents/metadata of the message being analyzed is likely to be exhibited together and this probability is utilized to determine a risk component score.

Therefore, in some embodiments, a risk is identified by determining the conditional probability of a set of message aspects, given the past observations relating to the sender, the sender's organization, and of users in general. It may also take into consideration the meaning of mismatches, e.g., upgrades and potential changes from one phone to another. IP addresses and associated IP traces may also be taken into consideration, along with data about the common case for the considered user. Any aberration is identified, and a risk score is computed from the conditional probability of the observations. This may be done by comparing the probability to at least one threshold and assigning an associated risk score, by algebraically converting the probability to a risk score, or a combination of these approaches. The risk score is also affected by the exposure of the recipient to attacks, message content, and other risk identifiers.

In an illustrative example, Alice and Bob regularly communicate with each other using email. For roughly 75% of the messages Alice sends to Bob, the message emanates from a mail client with mime version “1.0 (Mac OS X Mail 10.1\(3251\))” and with additional characteristics such as those shown in the “Mac” column of the table above. For roughly 25% of the messages Alice sends to Bob, the message emanates from a mail client with mime version “1.0” and with additional characteristics such as those shown in the “Android ph” column of the table above. Bob's system receives an email E1 from Alice that is sent from a mail client with characteristics such as those shown in the “Mac” column of the table above. This is considered consistent with Alice's past sending behavior, and E1 is delivered. Bob's system receives an email E2 from Alice, that is sent from a mail client with characteristics such as those shown in the “Mac” column of the table above, except that the message is associated with mime version “1.0 (Mac OS X Mail 9.1\(3121\)).” This would correspond to a downgrade of Alice's system, as opposed to an upgrade. This is considered unusual, and a security action is initiated. The contents of E2 is scrutinized, both using an automated tool and using a manual audit by an admin, after which it is concluded that the message is safe, and is delivered. The new mail client characteristic is entered in a record associated with Alice, along with details relating to the message and its treatment. Bob's system receives an email E3 from Alice that is sent from a mail client with characteristics such as those shown in the “iphone” column of the table above. This is not an entry that is associated with Alice's past observations. In addition, it is known that Alice has an Android phone. This causes an escalation of the message scrutiny. The message is found to have content matching a content risk filter. The determination is that E3 is at high risk of being a message that comes from an ATOed system. An SMS verification message is sent to Alice's previously recorded phone number, and Alice's record is marked as being at high risk of being corrupted. E3 is not delivered, but Bob receives a message saying that a suspect message has been blocked by the system, and that the message came from Alice's account. Cindy also knows Alice, and Cindy receives an email E4 from Alice. The system looks up Alice's record and determines that her account is considered at high risk of being corrupted. The message E4 is automatically scrutinized, but does not match any rule associated with a high risk. Therefore, E4 is marked up with a warning “This message comes from an account that is temporarily associated with risk. Please proceed with caution. Before performing any sensitive action, please verify with the owner of the account.” However, it turns out that Alice really did send E3, from her new phone. Accordingly, she confirms that E3 was sent by her. The security system delivers E3 to Bob, and replaces the marked-up version of E4 in Cindy's inbox with the “clean” version of E4. The system also removes the “corrupted” flag in the record associated with Alice, and records the header configuration associated with E3 in the record. The system does not know whether Alice will still use her Android phone. However, it knows from the response to the verification challenge—which was sent from an iPhone—that the phone number on record for Alice is likely to be associated with an iPhone, but that the phone associated with the “Android ph” device may also be used. Later on, Bob's system receives an email E5 from Alice, not matching any of the previously recorded headers, but instead matching a header configuration commonly used by previously observed scammers. The message scrutiny is escalated, and automated scrutiny indicates that E5 contains a zip file that, when decompressed in a sandbox, contains a malicious file. The system concludes that Alice's device has been corrupted, and adds information about this in the record associated with Alice. A notification message is sent to Alice using SMS. However, Alice does not respond to it. The security system initiates a notification to Alice's mail service provider, indicating that it is likely that Alice's system has been corrupted. Alice's mail service provider reviews the activity associated with Alice's account, and concludes that it is likely that Alice's account has been taken over. They automatically block access to the account and initiate an effort to allow Alice to regain access to her account. Several days later, this completes. Dave has not interacted with Alice in the past, but receives an email E6 from Alice. The system looks up Alice's record and determines that her account is considered at high risk of being corrupted. The message E6 is automatically scrutinized. It is found that it is identical to a message sent in the past from corrupted accounts. Therefore, E6 is not delivered. Alice's account sends another email E7 to Bob, which is trapped by the security system. It does not have content found to be dangerous, and so, is delivered along with a warning “This email was sent from a sender that may be hacked. Please proceed with caution.” The system generates an inquiry to Alice's mail service provider whether the recovery has completed; when the mail service provider responds that Alice's system has been recovered, the flag associated with corruption in Alice's record is set to “none,” and the modified message is searched for. However, the security system cannot find it, and so, does not replace it with its clean version E7.

In various embodiments, in addition to using historical observations associated with the sender of the message to analyze the security risk of the message, other types of security risk analysis are performed in determining a total risk score associated with the message. For example, in addition to one or more risk component scores for risk factors determined using past observations, one or more other risk component scores are determined for other types of security risk analysis. The various different types of risk component scores are combined to determine a total risk score. Examples of the other types of security risk analysis include a virus threat detection, a malware threat detection, identification of risky content type (e.g., executable, file attachment, link to a website that requests login information, content that requests OAuth authentication, etc.) included in the message, analysis of a number and type of recipients of the same message, and determining whether the sender from a domain with a Domain-based Message Authentication, Reporting and Conformance (DMARC) policy passes Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) validations.

If at 206 it is determined that the message was received from a sender that does not have an established relationship with the intended recipient of the message, at 210, the message is analyzed for security risks using general factors associated with the sender of the message. For example, the message is analyzed to identify security risks without the benefit of observations of previous communication history between the sender of the message and the intended recipient of the message. However, generalized past observations about messages from the sender (e.g., generalized based on messages for other recipients from the sender) and/or an organization/domain of the sender is utilized to analyze a message content keyword, a mail user agent, a time zone, an IP address, an X-header, supported character sets, a message signature, and any other information about the message to determine whether the message is sufficiently different from the generalized past observations to indicate a security risk. In some embodiments, analyzing the message for security risks includes performing a plurality of analyses and determining a risk component score for each of the analyses. These component scores are then combined (e.g., added, weighted then added, averaged, etc.) to determine an overall risk score. In some embodiments, each of the component scores is associated with one or more specific types of risk and a separate total score is calculated for each of the different types of risk based on its associated component scores.

In some embodiments, in 210, it is determined whether the electronic message account of the sender of the message is likely an independently controlled account. For example, an account that belongs to a large organization that closely controls who can send a message via its domain is not an independently controlled account whereas a personal email message account is an independently controlled account. In response to a determination that the electronic message account of the sender of the electronic message is likely an independently controlled account, the message is analyzed to determine whether the message is an automatically generated message. For example, a header of the message is analyzed to determine whether the message was automatically generated using a script or a program. In response to a determination that the message is an automatically generated message, a risk component score of the message is set to be high enough to warrant performing a security action.

In various embodiments, examples of other types of analyses performed in 210 include one or more of the following: a virus threat detection, a malware threat detection, identification of risky content type (e.g., executable, file attachment, link to a website that requests login information, content that requests OAuth authentication, etc.) included in the message, analysis of a number and type of recipients of the same message (e.g., attacker may prefer targeting people one by one, to avoid detection), and determining whether the sender from a domain with a Domain-based Message Authentication, Reporting and Conformance (DMARC) policy passes Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) validations.

At 212, based on a result of the analysis, a security action is performed, if applicable. In some embodiments, either in 208 or 210, one or more security risk scores are determined and based on these score(s), a security action is selected among different security action options. The selected security action is performed. For example, a security risk score may indicate that the message is of very low risk (e.g., risk score is below a first threshold) and the message is fully allowed to be accessed by the intended recipient (e.g., allow the message to a message inbox of the intended recipient). If not, the security risk score may indicate that the message is of medium risk (e.g., risk score is above the first threshold but below a second threshold) and the message is modified to include a warning prior to being allowed to be accessed by the intended recipient (e.g., allow the modified message to a message inbox of the intended recipient). Otherwise, the security risk score may indicate that the message is of high risk (e.g., risk score is above the second threshold) and the message not allowed to be accessed by the intended recipient (e.g., send the message to an administrator for further analysis). If there exists a different security risk score for different types of security risks, each different type of score may be analyzed separately and/or analyzed in combination to determine which level, if any, of security action is to be performed.

Examples of the security action (e.g., selected from the list based on a risk score and/or specific type of threat or information detected in the message) include the following: sending a message to the sender to determine whether it automatically forwards responses; generating a security challenge sent to the sender (e.g., if this is not responded to, it is indicative of higher risk and if it is responded to in an anomalous manner, that is also indicative of higher risk); generating a challenge sent to an alternative account of the sender (e.g., another email address associated with the sender); sending an SMS message to a phone number associated with the sender; placing an automated call to a phone number associated with the sender (e.g., requesting a confirmation or a response from the user associated with the account the security challenge is sent to); performing additional automated scrutiny of the message (e.g., including its content portion); performing additional manual scrutiny of the message (e.g., including its content portion); quarantining the message; blocking the message; delivering the message; augmenting the message to reduce the risk associated with it (e.g., modifying its attachments); analyzing attachments of the message by attempting to execute them in a sandbox or virtual machine; adding a warning to the message prior to allowing the message to be access by the intended recipient; and moving the message to a special folder identifying its higher risk.

In some embodiments, a software component is introduced at a protected sender side, the software component encoding information in a header element for recipients to observe. By letting the information be a function of a key that is shared between the sender side and the observer side, a message authentication code can be implemented. Alternatively, asymmetric cryptography such as digital signatures can be used for the authentication, as can streamed authentication methods such as TESLA or other hash chain constructions. The authenticated data may be an empty field, a portion of the message, a counter, data relating to the transmission time, and information about the context of the message creations, such as the IP address from which the message was submitted. It is desirable that the data is encrypted or that obfuscation methods are used; alternatively, proprietary formats can be used to complicate decoding by an unauthorized observer.

Attackers gaining access to email accounts typically perform searches to identify high-value contacts, e.g., by searching the mailbox for keywords indicating financial services, high-profile connections, and more. If a company wishing to limit the damage associated with the corruption of the accounts and machines of external associates were able to send messages that “self destruct,” the damages associated with the corruption may be mitigated. One example of achieving self-destructing emails includes replacing message content with a hyperlink. The outgoing mail server can automatically identify high-risk emails (based on content, recipient, and internal policies) and rewrite the content with a hyperlink that, when clicked, would give the recipient access to the content. This access can be controlled; it can be limited in terms of time after receipt (e.g., 3 days), it can require a password or other authenticating action, it can be conditional on an action not yet having been observed (e.g., no response has been received to this email yet), etc.

Another example of achieving self-destructing emails includes placing an email representing one or more archived messages in a special folder. If the associated account is controlled by the security service provider, received emails can be archived in a password-only storage after some time. This can be implemented by placing an email representing one or more archived messages in a special folder, the email having a hyperlink leading to a password-protected server giving full access to all archived emails. (The password protected server can limit access based on volume, IP address, etc.) There can be a policy determining what messages get auto-archived, based on sender, content and other indicators. The auto-archiving feature can depend on having observed a high-risk action, e.g., a user opening a risky email, following a link, and entering a text (which may be a password), or accessing the mailbox from an unknown computer.

FIG. 3 is a flowchart illustrating an embodiment of a process for analyzing individual aspects of a message for security threats. The process of FIG. 3 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 3 is performed in 208 and/or 210 of the process of FIG. 2.

At 302, content included in a message is analyzed to determine one or more content risk component scores of the message. For example a message body of the message is analyzed to identify whether it includes one or more text/keywords from a list of keywords associated with a security risk. A keyword may be included in the list because the keyword identifies conversations about sensitive content that could cause harm to the recipient if the recipient replies or takes action related to the keyword. For example, keywords “social security number,” “password,” “login,” “wire instructions,” “confidential,” “payment,” “ID,” obfuscated text, etc. may be included in the list of keywords associated with a security risk. A content risk component score is determined based on whether the message includes a text/keywords from the list of keywords (or associated synonyms or related words) associated with a security risk. For example, an increase in the number (e.g., of different types) and degree of risk of security risk keywords included in the message increases the associated content risk component score.

If the sender of the message has been determined to have an established relationship with the recipient of the message, past observations about messages sent by the sender are utilized in determining the content risk component score based on whether the message includes a text/keywords from the list of keywords (or associated synonyms or related words) associated with a security risk. For example, if it is common for a sender to communicate with the recipient about invoices and payment, an inclusion of a keyword related to invoices and payment would not cause the content risk component score to go up as much as compared to a situation in which it is not common for sender to communicate with the recipient about invoices and payment. Thus a contribution of a security risk keyword included in the message to the content risk component score is reduced if the same or similar keyword in scope and topic (e.g., keyword that is a synonym or topically related word) has been previously observed in previous messages from/to the sender to/from the recipient. The magnitude of this reduction may depend on the number and length of time the same or similar keyword in scope and topic has been previously observed in previous messages from/to the sender to/from the recipient.

In some embodiments, analyzing content included in the message to determine one or more content risk component scores of the message includes determining a content risk component score based on a message signature included in the message. For example based on past observations about types of email signatures typically included in email messages from the sender of the message, the associated content risk component score is increased if an expected signature is not included in the message (e.g., given other detected attributes of the message and the expected signature for the given other detected attributes) or if an unexpected signature is detected (e.g., never observed signature or included signature is an out of context signature that does not match an expected signature for the given detected attributes of the message).

At 304, entries included in a header of the message is analyzed to determine one or more header entry risk component scores of the message. Examples of the entries included in the analyzed header include one of more of the following: a mail user agent (i.e., MUA) identification, time zone identification, IP address, X-header, supported foreign language character sets of the sender, identification of automation or script that generated the message, or any other data included in the header (e.g., email header) of the message. For each header entry to be analyzed, a header entry risk component score is calculated based on a risk factor associated with the entry. For example, certain sender IP addresses and/or header entries, given what is known about the intended recipient (e.g., mismatch between sender and recipient) may increase the associated header entry risk component score. If the sender of the message has been determined to have an established relationship with the recipient of the message, past observations about headers of messages sent by the sender are utilized in determining the header entry risk component scores. For example, the header entry risk component score for a header entry is reduced if the same or similar entry (e.g., change in entry from a previously observed entry conforms to known change pattern) has been previously observed in previous message headers of messages from the sender of the message. The magnitude of this reduction may depend on degree of similarity and/or conformance to a typical pattern of change from previous observations.

In some embodiments, the header entry risk component score for a header entry is associated with whether the message is an automatically generated message. For example, one or more x-header entries identify whether the message was automatically generated. These entries may be identified based on keywords that identify that the message was generated using a script (e.g., generated using a PHP mailer/script). If the sender of the message has been determined to have an established relationship with the recipient of the message, past observations about headers of messages sent by the sender are utilized in determining the header entry risk component scores. For example, the header entry risk component score for a header entry identifying an automatically generated message is reduced if the sender typically sends automatically generated messages (e.g., including the same or similar header entry) to the recipient of the message. The magnitude of this reduction may depend on degree of similarity and/or conformance to a typical pattern of change from previous observations. The following are examples of header entries identifying an automatically generated message and these entries are identified by matching one or more search terms with at least a portion of the text of these entries: “X-Mailer: PHPMailer 5.2.23 (https://github.com/PHPMailer/PHPMailer)”; “X-Php-Originating-Script: 1002:srjvdopc.php(1189): runtime-created function(1): eval( )'d code(1): eval( )'d code.”

FIGS. 4A-4F show various examples of headers included in example email messages. The headers shown in the example message 402 of FIG. 4A are associated with messages that were received by a server, associated with the sender, located in the Pacific time zone. This can be seen by the “−0800” portion of the “date” header. In addition, the IP address of the received server will typically indicate a location consistent with the time zone. However, in the message below, the date header indicates a “+0900” time zone, which is associated with Japan. Also note that the subject line indicates UTF-8, which is associated with a sender using multiple character sets.

The example message 404 in FIG. 4B shows an IP address “94.245.18.21”, which is consistent with the expected location of the sender (Denmark and Sweden.) The X-header identifies the sending device as an “iPhone”, which is also consistent with the sender's historical headers.

The example message 406 in FIG. 4C shows a response to a Craigslist advertisement, for an ad associated with Mountain View, Calif. The time zone of the server where the message was received is indicative of potentially having been sent from Africa. This is in spite of the sender using a “gmail” account. The reason is that Google has servers in many locations, and the closest location to the sender was likely in use, for efficiency reasons. This is, of course, a sign of risk in the context of a local transaction, where one would expect the sender to be local. But more importantly, had the sender's “gmail” account commonly been associated with an IP address and time zone in, say, Missouri, and suddenly becomes associated with an IP address and time zone in Africa, then this is indicative of an account take-over.

The example message 408 in FIG. 4D is another response to the same advertisement. This sender is also located in a location other than California. The example message 410 in FIG. 4E has an unusual X-header. If an email is received, appearing to come from this sender, but not having this X-header, then there is a good chance that the message is from an account that has been compromised.

The example message 412 in FIG. 4F identified as having been sent using a sending server four time zones away from the receiving server, from a user associated with charset ISO-8859-1. The system can identify what charsets are commonly used by legitimate senders sending traffic to a receiving user or organization, as well as the distribution in time zone difference. When a previously not observed user sends a protected user a message, it can be determined if the headers are consistent with those of previously received good traffic, whether they match high-risk senders, or neither. A risk score can be computed based on the headers and the extent to which they match known good and known bad traffic. In one scoring example, a score of 100 is generated when all headers match those of the sender's past headers. A score of 35 is computed for another previously non-observed sender that fails to match any other previous sender. A score of −50 is computed for a previously observed sender whose historical headers are very different from the currently observed headers, including different X-headers, a different time zone, and a different IP trace. A score of −10 is generated for a previously not observed sender whose headers match those of previous high-risk senders, but which do not match those of previous low-risk senders to the recipient.

At 306, one or more other security analyses is performed to determine one or more other security risk component scores. This step may be optional in various embodiments. Examples of other types of analysis performed include one or more of the following: a virus threat detection, a malware threat detection, identification of risky content type (e.g., executable, file attachment, link to a website that requests login information, content that requests OAuth authentication, etc.) included in the message, analysis of a number and type of recipients of the same message, and determining whether the sender from a domain with a Domain-based Message Authentication, Reporting and Conformance (DMARC) policy passes Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) validations.

In some embodiments, performing a security analysis includes identifying which domains and subdomains are used to send legitimate traffic, e.g., by recording what subdomains/domains are used to originate large volumes of emails, and which are not known to be spam or fraud email. For example, “large amounts” may mean greater than a threshold value, such as 100 emails per week, or at least 0.1% of the traffic associated with a particular domain, or any traffic that is not known to be good, e.g., by being associated with correct SPF and/or DKIM data in the headers. Next, any traffic associated with a subdomain and domain that is not on the list of subdomains/domains that are known to send legitimate traffic is flagged, and, depending on a policy and/or contents, quarantined, blocked, marked up, or escalated for additional scrutiny.

In some embodiments, a security analysis involves web bugs, which are also referred to as web trackers. Some users set their browser preferences not to permit web bugs, whereas others do not. Therefore, the acceptance or rejection of a web bug is a trait that can be tracked. When it changes, that is indicative of risk. A web bug can be set to send a signal to the security service when the email or webpage in which it is integrated is rendered. The security service can send a message to a user considered at risk for having been corrupted, and then determine whether the web bug is operating in a manner that is consistent with past uses. The email can also demand that the user allows web bugs, after which user-specific information can be collected. If this is found to be inconsistent with past observations, the associated risk component score is increased.

At 308, one or more of the risk component scores are combined together to determine one or more combined security risk scores. Determining a combined security risk score includes statistically combining (e.g., adding together, weighting then adding together, averaging, weighted averaging, etc.) together the various risk component scores to determine a single combined risk score. In some embodiments, each of the risk component score(s) is associated with one or more specific types of risk. For example, risk component scores of each risk type are to be combined together by type to determine a total risk score for each specific risk type. For example, a security risk component score may be identified as being associated with a malware risk type and/or a phishing risk type and a combined security risk score for the malware risk type and a separate combined security risk score for the phishing risk type are calculated. In some embodiments, the combined security risk scores are utilized in determination of which security action to perform in 212 of FIG. 2.

FIG. 5 is a flowchart illustrating an embodiment of a process for analyzing aspects of a received message based on whether the message is from an independently controlled account of a sender. The process of FIG. 5 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 5 is performed in 208 and/or 210 of the process of FIG. 2. For example, the process of FIG. 5 is performed in response to a determination that the sender of the electronic message is not an established communication contact of the intended recipient of the message.

At 502, it is determined whether an electronic message account of a sender of the electronic message is likely an independently controlled account. For example, a message that was sent from an account that belongs to a large organization that closely controls who can send a message via its domain (e.g., web domain) is not an independently controlled account whereas a personal email message account is an independently controlled account.

In some embodiments, determining whether the electronic message account of a sender of the electronic message is likely an independently controlled account includes identifying a domain of an address (e.g., email address) of the sender of the message using the domain to identify using preconfigured data, whether the domain corresponds to independently controlled accounts. For example, certain domains are known to only allow not independently controlled accounts and any email address using these domains is identified as not an independently controlled account, while other domains are known to allow independently controlled accounts and any email address using these domains are identified as an independently controlled account. If the domain is unknown, the electronic message account of the sender is identified as an independently controlled account.

In some embodiments, the electronic message account of the sender of the electronic message is determined to be likely an independently controlled account based on a message traffic pattern associated with the electronic message account of the sender. For example, based on the volume of messages (e.g., large volume of message traffic above a threshold to/from the sender indicates that it is not an independently controlled account) and/or number/ratio of sent messages vs. received messages (e.g., a ratio of sent messages by the sender as compared to messages to the sender that is above a threshold indicates that it is not an independently controlled account where as a generally even ratio of sent vs. received indicates an independently controlled account), it is determined whether the electronic message account of the sender of the electronic message is likely an independently controlled account.

The notion of autonomy (e.g., independently controlled) of an account is important with respect to trust of the account. For example, a “gmail” account is autonomous (e.g., independently controlled). Even though many members share the same domain, there is no coherency in their actions. On the other hand, an official work organization email address of one employee is not autonomous from another employee of the same organization.

In a first example, consider Alice with an email address with the domain “Aliceco.com” that sends an email to Bob at an email address with the domain “Bobco.com.” Assume that the recipient Bob has a trust relationship with Alice (i.e., the recipient email address for Bob has sufficient interaction with the Alice email account). Then there is “personal trust” by the recipient directed to the sender. Assume that the recipient does not have a trust relationship with Alice, but somebody else at “Bobco.com” does. Then there is “organizational trust” by the recipient directed to the sender. Assume now that the recipient Cindy at “Bobco.com” has no trust relationship with Alice, but Cindy does have one with Dave at Aliceco.com. Then there is “personal transitive trust” by the recipient directed to the sender. This is because Alice and Dave both belong to “Aliceco.com”, and “Aliceco.com” is one autonomous entity—Alice and Dave are not two independent autonomous entities. Finally, assume that the recipient has no trust relationship with either Alice or Dave—or anybody else at “AliceCo.com”—but that somebody else at “Bobco.com” does. This shows “organizational transitive trust.”

In another example, consider Bob using his “gmail” personal account, sends an email to somebody at “Bobco.com.” Assume that the recipient has a trust relationship with Bob (e.g., the recipient email address has sufficient interaction with the Bob “gmail” email account). Then there is “personal trust” by the recipient directed to the sender. Assume that the recipient does not have a trust relationship with Bob's “gmail” account, but somebody else at “Bobco.com” does. Then there is “organizational trust” by the recipient directed to the sender. Assume now that the recipient has no trust relationship with Bob, but they do have one with Cindy's personal “hotmail” account. There is no trust—Bob and Cindy are not two independent autonomous entities when represented by their personal email addresses.

Thus if an email comes from a stranger whose email address is similar to a party the recipient trusts, then this is a high-risk scenario. Trust is different from more general reputation: one is not more likely to act on an email from a trustworthy but unknown organization than one of an email from a non-trustworthy and unknown organization. However, whether one has a good relationship with a sender matters more in terms of trust than whether others have a good relationship with the sender. Different kinds of trusts may be computed in different ways but also have different strengths. For example: strength(“personal trust”)>strength(“organizational trust”); strength(“personal trust”)>strength(“personal transitive trust”); strength(“personal transitive trust”)>strength(“organizational transitive trust”); strength(“organizational trust”)>strength(“organizational transitive trust”). In some embodiments, different weights can be assigned to these different types of trust to determine a risk of a message.

If at 502 it is determined that the electronic message account of the sender of the electronic message is likely an independently controlled account, at 504, the message is analyzed to determine whether the message is an automatically generated message. For example, a header of the message is analyzed to determine whether one or more x-header entries identify whether the message was automatically generated. These entries may be identified based on keywords that identify that the message was generated using a script (e.g., generated using a PHP mailer/script). The following are examples of header entries identifying an automatically generated message and these entries are identified by matching one or more search terms/keywords with at least a portion of the text of these entries: “X-Mailer: PHPMailer 5.2.23 (https://github.com/PHPMailer/PHPMailer)”; “X-Php-Originating-Script: 1002:srjvdopc.php(1189): runtime-created function(1): eval( )'d code(1): eval( )'d code.”

In some embodiments, determining whether the message is an automatically generated message includes identifying a likely scripted behavior associated with a protected account. For example, attackers typically configure accounts they have compromised to forward emails to another account, either selectively (such as based on the subject line, the sender, the thread, etc.) or for all emails. In addition, attackers sometimes configure compromised accounts so that selective emails are hidden, as will be described in more detail below. When attackers make such a configuration, then this affects the traffic patterns in a way that will be measured and detected by the security service. By monitoring the different mail folders, including the inbox, the sent box, the archive, the trash folder and other folders, the security service provider will detect forwarding behavior that is anomalous in that it has not been performed by the account owner before; that is selective; that is rapid; or that is to recipients that are considered high risk, or any combination of these. It also detects signs of scripted forwarding, which is indicated by a very short time period between receipt of an email and the forwarding of the email. In addition, the security service also looks for other indications of scripted behavior, such as an anomalous addition of an account in the bcc, which is considered anomalous by not having been performed previously, and/or which is considered scripted due to the time periods being very short. Another related detection method is to observe if the MUA of emails to some recipients of emails sent from the protected account are different than those of others, and considered high risk, e.g., due to information that is anomalous for the account owner.

If at 504 it is determined that the message is an automatically generated message, at 506, an automated message security risk component score associated with the message is set (e.g., set to a high value) to reflect a high security risk of the message (e.g., higher than otherwise). This security risk component score may be utilized in 212 of FIG. 2 and/or 308 of FIG. 3.

If at 502 it is determined that the electronic message account of the sender of the electronic message is likely not an independently controlled account or if at 504, it is determined that the message is not an automatically generated message, at 508, the automated message security risk component score associated with the message is set to reflect a low risk of the message (e.g., lower than otherwise).

FIG. 6 is a flowchart illustrating an embodiment of a process for wrapping encrypted content of the message. The process of FIG. 6 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In various embodiments, at least a portion of the process of FIG. 6 is performed in 212 of FIG. 2. For example, the process of FIG. 6 describes one of the security actions that is selected to be performed.

If a message includes encrypted content such as an encrypted email attachment, that encrypted content is often unable to be properly analyzed for security threats because analysis tools are unable to access the encrypted content. Hackers may take advantage of this by sending an encrypted malware attachment along with a password to be utilized by the recipient to open the attachment.

At 602, encrypted content included in a message is identified. Examples of the message include an email, instant message, a chat message, and any other forms of electronic messages. The message may be analyzed as a part of a computer security analysis and filtered to identify and neutralize security threats prior to allowing an intended recipient of the message to access the message. In some embodiments, each message received for delivery to its intended recipient associated with the system performing analysis of the message is to be analyzed. Examples of the encrypted content include: an encrypted executable file, an encrypted zip file, an encrypted text, an encrypted script, an encrypted image, and any other encrypted content. The encrypted content may have been identified by identifying content that requires a password to access it.

At 604, a wrapped version of the identified encrypted content is generated and the message is modified to include the wrapped version of the encrypted content instead of the original encrypted content. In some embodiments, wrapping the encrypted content includes embedding the encrypted content in an executable program. When the executable program is executed, the executable program requests a decryption password from the user and the encrypted content of the message is encrypted using the decryption password and the decrypted content is analyzed for security threats prior to allowing the user access to the decrypted content once the analysis has completed without identifying a security threat. If the security analysis identifies a threat, a user may not be allowed to access the decrypted content and the security threat may be logged or reported to an administrator.

For example, the wrapped version of an encrypted zip file is a file that, when opened, causes execution, whether of the file wrapper itself or of an associated application that the message recipient needs to download, and as the file wrapper is executed, it requests and receives a decryption key, PIN, or password from the user and uses the PIN or password to decrypt the wrapped zip file. This is done by executing the unzip program associated with the zip file, or a compatible version to this, providing the unzip program with the PIN or password provided by the user. It then automatically accesses the contents of the unzipped file and analyzes it for security risks. This can be done in a multiplicity of ways, including detonating each file, determining whether any of the files match an anti-virus signature, determining whether any of the files has executable code segments in it, etc. The same approach also works for messages with attachments that are encrypted pdfs, or any other encrypted file. In such cases, the execution of the wrapper file causes the decryption of the encrypted file, which is then analyzed for security risks, including executable code. If a file, independent of type, passes the verification, then the decrypted content is made available to the recipient. Thus, the user experience associated with the opening of wrapped encrypted content would be the same as that of unwrapped encrypted content, except for a delay during security analysis (e.g., in most cases this would be too short to detect).

At 606, the message with the wrapped version of the encrypted content is allowed to be delivered. For example, the modified message is allowed to be delivered to an email inbox (or other email categorization folder to be actively accessed by a user to obtain new messages, e.g., not a spam or trash folder) of an end recipient. Allowing the modified message to be delivered includes allowing an intended user recipient of the message to be able to access the wrapped encrypted content. When the recipient opens the wrapped encrypted content, the wrapper program requests a decryption key, decrypts the encrypted content using the decryption key, and initiates the security analysis of the decrypted content prior to allowing the recipient access to the decrypted content of the encrypted content. Any security analysis technique or method described in this specification may be performed on the decrypted content to analyze it for security threats. If a security threat is detected, the user may not be allowed access to the decrypted content and a security may be performed. The security action may include revoking access to the message, deleting the message, forwarding the message, reporting the message, further modifying the message, moving the message (e.g., to a different folder), preventing access to a portion of the message, providing an additional warning, and/or performing further analysis.

FIG. 7 is a flowchart illustrating an embodiment of a process for tracking trusted contacts. The process of FIG. 7 may be at least in part performed by analysis server 102, gateway 110 and/or message server 106 of FIG. 1. Examples of the contacts include email addresses, usernames, display names, user identifiers, an identification photo, and any other identifier that may be utilized to identify a sender or recipient of a message. At least a portion of the process of FIG. 7 may be performed in 206 of FIG. 2. For example, one or more steps of the process of FIG. 7 are performed to determine whether a sender of a message is a trusted contact or has an established relationship with the recipient of the message.

At 702, trusted contacts for a user (e.g., message recipient) are determined. The trusted contacts are to be utilized to identify potential message threats of the user. The trusted contacts identify one or more contacts (e.g., senders or recipients of messages) that are familiar to the user because the user has previously interacted with the contact and/or is aware of the contact. In some embodiments, determining the trusted contacts includes receiving a list of contacts for the user. For example, the user provides access to an address book of the user and information about contacts in the address book is received. In some embodiments, the trusted contacts include contacts that have been specifically identified by the user. In some embodiments, information about the trusted contacts is stored. For example, the trusted contacts for the user are stored in a database of trusted contacts. This database may track trusted contacts for a plurality of different users and allows trusted contacts to be retrieved for a specific identified user. The stored information of each contact may include one or more of the following: email address, associated name (e.g., display name), relationship identifier, identifying image (e.g., contact photo), username, instant message identifier, address, phone number, a measure of trust, a measure of message interaction, and any other identifier utilized to identify a sender or a receiver of a message.

In some embodiments, the trusted contacts for the user are at least in part determined automatically based on contacts detected in messages sent or received by the user. For example, by observing and analyzing message traffic of the user and patterns of message recipients and senders, contacts that receive messages from the user and contacts that send messages to the user can be determined and correlated to infer and determine trust, frequency, and/or importance of interaction and relationship between the user and the contact to identify one or more of these contacts as a trusted contact. In one example, if a threshold number of messages has been sent to and from a contact for a user, the contact is identified as a trusted contact and added to a stored list of trusted contacts for the user. Information about the messages of the user utilized to at least in part automatically determine the trusted contacts has been obtained for analysis by receiving at least a portion of the messages (e.g., a copy of the entire message, email addresses and names of contacts of messages, etc.) for analysis (e.g., at server 102 from server 104, gateway 110, or server 106 of FIG. 1).

At 704, trusted contacts for a group that the user belongs to are determined. For example, trusted contacts for the users included in the group are aggregated to identify patterns among the aggregated trusted contacts. Examples of the group include a network domain that the user belongs to (e.g., domain of the email address of the user), an entity/organization that the user belongs to (e.g., company that the user belongs to), a message service provider, or an authority that controls a message account of the user. In some embodiments, determining trusted contacts for the group includes aggregating information about a trusted contact and/or a network domain of one or more trusted contacts from information gathered for different users for the trusted contact and/or the network domain. In some embodiments, determining the trusted contacts for the group includes storing information about the trusted contacts in a data storage structure and associating this stored information with the group.

At 706, determined trusted contacts are utilized to identify potential message threats for the user. For example, the contacts in the trusted contacts may be utilized as a proxy for contacts that are familiar to the user and if an attacker tries to attack the user using an impersonation attack, contact being impersonated likely corresponds to one of the contacts in the list of trusted contacts of the user. By determining a measure of similarity between a sender identifier of a message received for the user with each identifier of each trusted contact, messages attempting impersonation attacks may be identified. For example, if the email address of the sender matches an email address of a trusted contact, the message is to be trusted and not filtered but if the email address of the sender does not match any trusted contacts but is similar to an email address of a trusted contact, the message is identified as potentially a part of an impersonation attack and filtered. Filtering the message may include blocking the message, quarantining the message, further analyzing the message, and/or modifying the message (e.g., insert a warning).

FIG. 8 is a flowchart illustrating an embodiment of a process for determining information about a message sender to be utilized to assess a risk of a message. The process of FIG. 8 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 8 is performed in 702 and/or 704 of FIG. 7. For example, the process of FIG. 8 may be repeated for each trusted contact determined for a user (e.g., for each contact in a list of trusted contacts stored in a repository) and stored in a data structure that tracks the trusted contacts. The process of FIG. 8 may be repeated periodically. At least a portion of the process of FIG. 8 may be performed in 206 FIG. 2.

At 802, a measure of global reputation for a sender is determined. The sender may correspond to a group of individual sender addresses in some embodiments or an individual sender address in other embodiments. For example, the sender corresponds to a trusted contact in a list of trusted contacts determined using the process of FIG. 7. In another example, the sender corresponds to a domain of a trusted contact identified using the process of FIG. 7. In some embodiments, the sender is a network/Internet domain and the measure of global reputation corresponds to all individual sender addresses of the domain. For example, the determined measure of global reputation of a domain is applicable to all email addresses within the domain. In some embodiments, the measure of global reputation for a sender is specific to an individual sender address. For example, the measure of global reputation for a sender is specific to an email address. In some embodiments, the sender is one of a plurality of senders in a list of senders to be analyzed to predetermine its global reputation. The list of senders may be automatically determined from one or more previously received messages (e.g., senders of previously received messages) and/or specified by a user/administrator. The measure of global reputation of the sender is applicable for a plurality of recipients of a message from the sender while a measure of local reputation of the sender is specific to a particular recipient domain or individual recipient address of a message from the sender.

In some embodiments, the measure of global reputation indicates a general measure that a sender is likely to send a message that is of value to a recipient of the message. For example, a higher measure of reputation indicates a higher likelihood that any message sent by the sender will not include malicious and/or undesired content (e.g., message will not include a virus, malware, spam, etc.). In some embodiments, determining the measure of global reputation includes determining one or more component factors to be utilized to determine a total measure of global reputation. For example, each determined component factor may be utilized to add, subtract, or multiply a value to/from the measure of global reputation to determine the total measure of global reputation. In one example, determined component factor values are summed to determine the global reputation measure. In some embodiments, machine learning or another automated process is utilized to determine the measure of global reputation based on gathered/generated information about the sender of the global reputation.

In some embodiments, determining the measure of global reputation for the sender includes determining whether an identifier of the sender (e.g., domain name, subdomain, IP address) is included in a list of senders known to be reputable. For example, there exists a predetermined list of entities and their associated identifiers that are known to be reputable and if the identifier of the sender is included in the predetermined list, the resulting measure of global reputation would be higher than otherwise. In some embodiments, determining the measure of global reputation for the sender includes determining whether an identifier of the sender is included in a list of senders known to be not reputable. For example, there exists a list of IP addresses associated with a server that are known to be commonly utilized to send spam messages and in the event an IP address associated with the sender is included in this list, the determined measure of global reputation would be lower than otherwise. Whether the identifier of the sender is included in the list of senders known to be reputable or not reputable may be utilized to determine a component factor value utilized in determining the measure of global reputation of the sender.

In some embodiments, determining the measure of global reputation for the sender includes determining a category of business associated with an entity of the sender. For example, businesses in certain categories (e.g., banking) may be associated with a higher reputation than businesses in other categories (e.g., digital marketing). In some embodiments, using a list/database that maps an identifier of a sender to a known associated category/line of business, the category/line of business associated with the sender is determined, if applicable (e.g., the line of business for some senders may be unknown or not applicable). A reputation component factor (e.g., value to be added, multiplied, or subtracted to/from total score measure) associated with the identified category/line of business may be utilized in determining the measure of global reputation.

In some embodiments, determining the measure of global reputation for the sender includes analyzing domain registration history and Domain Name System (i.e., DNS) activity of the sender. For example, a sender that is typically reputable will register a domain name far ahead of time prior to the use of the domain while a less reputable sender will likely temporarily utilize a domain for a short period of time prior to moving on to another domain and will register a domain within a short amount of time prior to the use of the domain. In some embodiments, determining the measure of global reputation includes utilizing a component factor value determined based on the domain registration history and DNS activity analysis (e.g., add, multiply, subtract, etc. using the factor value). For example, the factor value is based at least in part on a length of time since registration of a domain of the sender, an amount of time between registration of the domain and a first use of the domain to send a message, Internet content (e.g., webpage) located at a URI utilizing the domain of the sender, an entity that registered the domain of the sender, etc.

In some embodiments, determining the measure of global reputation for the sender includes analyzing historical statistics and/or a history of messages sent by the sender. For example, there exists one or more repositories of previous messages sent by the sender and the repositories of messages are analyzed to determine one or more of the following: an amount/volume/rate of messages sent by the sender, a number/breadth of different recipients (e.g., number of different domains of recipients, number of different email addresses of recipients, etc.) of messages from the sender, a distribution of messages sent to different recipients, a length of time the sender has been sending messages, a regularity that the sender has been sending messages (e.g., difference in average number of messages per time period), a type of content of messages sent by the sender, a difference/regularity between content of messages sent by the sender, amount/rate of content opened/viewed by recipients of messages sent by the sender, a number/rate of messages sent by the sender that have been identified as malware, spam, and/or a virus by an analysis/filter/scanner tool, etc. The historical analysis may be performed using machine learning. In some embodiments, based on the historical analysis, a historical analysis component factor value is determined and the historical analysis component factor value is added, multiplied, subtracted, etc. to determine the measure of global reputation for the sender.

In some embodiments, determining the measure of global reputation for the sender includes determining whether an identifier of the sender is similar to an identifier of another sender. For example, it is determined whether a domain of the sender is similar to a domain of another sender. Determining whether an identifier of the sender is similar to an identifier of another sender may include determining whether an identifier of the sender is similar to an identifier of another sender with a relatively high measure of global reputation. For example, an attempt to confuse a recipient by utilizing a domain that is only slightly different from a highly reputable domain is detected. In some embodiments, based on the similarity analysis, a similarity component factor value is determined and the similarity component factor value is added, multiplied, subtracted, etc. to determine the measure of global reputation for the sender.

In some embodiments, the global reputation for the sender is determined by combining (e.g., averaging, adding, etc.) measures of local reputation (determined in 804) for the sender determined for each recipient of a group of different recipients to determine a combined measure (e.g., combined score).

At 804, a specific measure of local reputation for the sender is determined for each of one or more different recipients. For example, although the measure of global reputation determined in 802 may apply to any recipient that receives a message from the sender, the specific measure of local reputation is only applicable to a specific recipient. In one example, one recipient may have a different relationship to a sender than another recipient (e.g., existing business relationship) and interest/relevance of a message from the sender may be different for different recipients and this difference is reflected as a different measure of local reputation for each different recipient. In some embodiments, the one or more different recipients include the user in 702 of the process of FIG. 7. In some embodiments, the recipient represents a network/Internet domain and the measure of local reputation corresponds to all individual recipients of the domain. For example, the determined measure of local reputation for a recipient domain is applicable to all recipient email addresses within the domain. In some embodiments, the measure of local reputation for the sender is specific to an individual recipient address.

In some embodiments, the recipient is one of a plurality of recipients in a list of recipients to be analyzed to predetermine the local reputation of the sender for the specific recipient. The list of recipients may correspond to the recipients/email servers accessible (e.g., list of email domains being managed) by an analysis server. In some embodiments, each email server of each recipient performs its own determination of its measure of local reputation for the sender. In some embodiments, the measure of local reputation is determined dynamically. For example, when a recipient receives a message from the sender, the recipient determines the measure of local reputation for the sender.

In some embodiments, the measure of local reputation indicates a measure that the sender is likely to send a message that is of value to a specific recipient. For example, a higher measure of reputation indicates a higher likelihood that an authentic message sent by the sender will not include malicious and/or undesired content (e.g., message will not include a virus, malware, spam, etc.) for the specific recipient. In some embodiments, determining the measure of local reputation includes determining one or more factors to be utilized to determine a total measure of local reputation. For example, each determined factor may be utilized to add, subtract, or multiply a value to/from the measure of local reputation to determine the total measure of local reputation. In some embodiments, machine learning or another automated process is utilized to determine the measure of local reputation based on gathered/generated information about the sender.

In some embodiments, determining the measure of local reputation for the sender includes determining whether an identifier of the sender (e.g., domain name, subdomain, IP address) is included in a list of senders known to be reputable for the recipient. For example, there exists a predetermined list of entities and their associated identifiers that are known to be reputable for the specific recipient. If the identifier of the sender is included in the predetermined list, the resulting measure of local reputation would be higher than otherwise. In some embodiments, determining the measure of local reputation for the sender includes determining whether an identifier of the sender is included in a list of senders known to be not reputable for the specific recipient. For example, there exists a list of IP addresses associated with a server that are known to be commonly utilized to send spam messages and in the event an IP address associated with the sender is included in this list, the determined measure of local reputation would be lower than otherwise. Whether the identifier of the sender is included in the list of senders known to be reputable or not reputable may be utilized to determine a component factor value utilized to determine the measure of local reputation of the sender.

In some embodiments, determining the measure of local reputation for the sender includes determining a category of business associated with an entity of the sender with respect to a property of the specific recipient. For example, businesses in certain categories (e.g., banking) may be associated with a higher reputation than businesses in other categories (e.g., digital marketing) with respect to the category of business of the recipient (e.g., recipient is also in banking). In some embodiments, a reputation component factor (e.g., value to be added, multiplied, or subtracted to/from total score measure) associated with the identified category/line of business with respect to the recipient may be utilized in determining the measure of local reputation

In some embodiments, determining the measure of local reputation for the sender includes analyzing domain registration history and Domain Name Service (i.e., DNS) activity of the sender with respect to a property of the recipient. For example, it may be typical for certain recipients to be in communication with senders that utilize a domain for a short period of time while for other recipients it is not typical. In some embodiments, determining the measure of local reputation includes utilizing a component factor value determined based on the domain registration history and DNS activity analysis (e.g., add, multiply, subtract, etc. using the factor value). For example, the factor value is based at least in part on a length of time since registration of a domain of the sender, an amount of time between registration of the domain and a first use of the domain to send a message, Internet content (e.g., webpage) located at a URI utilizing the domain of the sender, an entity that registered the domain of the sender, etc.

In some embodiments, determining the measure of local reputation for the sender includes analyzing historical statistics and/or a history of messages sent by the sender for the specific recipient domain or individual recipient addresses. For example, there exists a repository of previous messages sent by the sender to the recipient (e.g., messages stored at a recipient message server of the recipient) and the repository of messages is analyzed to determine one or more of the following: an amount/volume/rate of messages sent by the sender, a number/breadth of different individual users of the recipient (e.g., number of different email addresses of recipient) that received messages from the sender, a distribution of messages sent to different individual users of the recipient, a length of time the sender has been sending messages to the recipient, a regularity that the sender has been sending messages to the recipient (e.g., difference in average number of messages per time period), a type of content of messages sent by the sender to the recipient, a difference/regularity between content of messages sent by the sender to the recipient, amount/rate of content opened/viewed by the recipient of messages sent by the sender, a number/rate of messages sent by the sender that have been identified as malware, spam, and/or a virus by an analysis/filter/scanner tool, etc. The historical analysis may be performed using machine learning. In some embodiments, based on the historical analysis, a historical analysis component factor value is determined and the historical analysis component factor value is added, multiplied, subtracted, etc. to determine the measure of local reputation of the sender for the recipient.

In some embodiments, determining the measure of local reputation for the sender includes determining whether an identifier of the sender is similar to an identifier of another sender and/or an identifier of the recipient. For example, it is determined whether a domain of the sender is similar to a domain of another sender or a domain of the recipient. Determining whether an identifier of the sender is similar to an identifier of another sender or the recipient may include determining whether an identifier of the sender is similar to an identifier of another sender with a relatively high measure of local reputation for the recipient. For example, an attempt to confuse a user by utilizing a sender domain that is only slightly different from a highly reputable domain for the recipient is detected. In some embodiments, an attempt to confuse a user by utilizing a sender domain that is only slightly different from a domain of the recipient is detected (e.g., detect trying to mimic an intra-organization message). In some embodiments, based on the similarity analysis, a similarity component factor value is determined and the similarity component factor value is added, multiplied, subtracted, etc. to determine the measure of local reputation of the sender for the recipient.

At 806, a global sender model for the sender is determined. For example, the sender model may be utilized to determine whether a message that indicates a sender was actually sent by the indicated sender. In some embodiments, the sender model identifies one or more properties that are characteristic of a message that is sent by the sender. In some embodiments, the sender model associates the sender with one or more IP addresses of message servers that are known or authorized to send messages for the sender. For example, an IP address of a last server that sent a message is a reliable identifier that indicates an origin of the message and this IP address information is utilized to determine whether the last server that sent the message corresponds to the sender identified in the message. In some embodiments, the sender model includes directly provided information. For example, a list of IP addresses of servers that send messages for the sender is received from a user, the sender, or a published source of information about the sender. In some embodiments, at least a portion of the sender model is determined using message authentication/validation information about the sender. For example, IP addresses associated with a domain of the sender are obtained using standardized message authentication/validation systems (e.g., using Domain-based Message Authentication (DMARC), DomainKeys Identified Mail (DKIM), Sender Policy Framework (SPF), etc.).

In some embodiments, the sender model is automatically determined. For example, using one or more repositories storing messages received from the sender, a list of server IP addresses authorized or historically known to send messages for the sender is automatically determined. The one or more repositories may include a message store of a plurality of different recipients (e.g., different recipient message servers managed by an analysis server) and external message sources (e.g., information from third-party sources that gather message information). Machine learning may be utilized to automatically detect sources of and properties that are characteristic of authentic messages from the sender using historical information about messages previously sent by or on behalf of the sender. In some embodiments, an IP address is automatically associated with the sender in the sender model if it is detected that a message likely/verified to be sent by the sender was sent from the IP address. In some embodiments, before an IP address is associated with the sender, the sender must have sent a message from the IP address at least a threshold number of times. In some embodiments, a message is determined to have been sent by the sender if the message was validated using an email authentication/validation system and/or sent from the IP address that has been utilized to send messages identified as being sent by the sender for at least a threshold amount of time and/or a threshold number of times. Other factors may be utilized to verify whether a message in a repository was sent by the sender prior to using IP address information of the message in determining the sender model of the sender.

In some embodiments, an IP address is only added to the sender model to be associated with the sender in the event the IP address has been determined to be consistent for the sender. For example, a history of senders associated with the IP address is analyzed and the IP address is only associated with the sender if the IP address is not already associated with another sender, has not been associated with another sender for at least a threshold amount of time, and/or the number of different previous senders that have been with associated with the IP address is less than a threshold number.

In some embodiments, the sender model identifies one or more blocks of adjacent IP addresses that are authorized or historically known to send messages on behalf of the sender. These blocks of adjacent IP addresses may be owned or be under the control of a single administrative entity or domain or may be detected to exhibit similar sending behavior. The block of adjacent IP addresses may be specified by a user, a sender, an administrator, and/or a source of published information about the sender or a provider authorized by the sender. In some embodiments, one or more blocks of adjacent IP addresses to be included in the sender model are automatically determined. For example, once one or more IP addresses within a block of adjacent IP addresses have been associated with a sender model, the entire block of adjacent IP addresses may be associated with the sender model if they are determined or expected to exhibit similar message sending behavior.

In some embodiments, the sender model identifies a network/Internet domain that is common to all hostnames of servers that are authorized or historically known to send messages on behalf of the sender. The servers that share this network/Internet domain may be owned or be under the control of a single administrative entity or domain or may be detected to exhibit similar sending behavior. The network/Internet domain may be specified by a user, a sender, an administrator, and/or a source of published information about the sender or a provider authorized by the sender. In some embodiments, the network/Internet domain to include within the sender model is automatically determined. For example, once one or more IP addresses whose hostnames share an organizational network/Internet domain have been associated with a sender model, all servers whose hostnames share that network/Internet domain may be associated with the sender model if they are determined or expected to exhibit similar message sending behavior.

In some embodiments, the sender model identifies one or more autonomous system numbers (i.e., ASN) associated with servers that are authorized or historically known to send messages on behalf of the sender. For example, an ASN identifies a collection of IP routing prefixes owned by or under the control of a single administrative entity or domain and the ASN associated with the sender is specified in the sender model. The ASN associated with the sender may be specified by a user, a sender, an administrator, and/or a source of published information about the sender. In some embodiments, the ASN associated with the sender is automatically determined. For example, once one or more IP addresses associated with a sender model have been identified, the ASN(s) associated with the IP addresses may be associated with the sender if they are determined or expected to exhibit similar message sending behavior.

The determined or specified sender model associated with a sender may be stored in a data structure such as a list, a database, a table, or any other data structure that can be indexed based on an identifier of the sender.

At 808, a specific sender model for the sender is determined for each of one or more recipients, if applicable. For example, although the sender model determined in 806 may apply to any recipient, the specific sender model is only applicable to a specific recipient. In one example, one recipient may receive a message from the sender via a different set of sending message servers than another recipient due to geographical location differences between the recipients. In some embodiments, the recipient represents a network/Internet domain and the recipient specific sender model corresponds to all individual recipients of the domain. In some embodiments, the recipient specific sender model is specific to an individual recipient address. In some embodiments, the recipient specific sender model is determined in a similar manner as the sender model determined in 806 except for utilizing data specific to the recipient rather than various different recipients. For example, using a repository storing messages received from the sender to only the recipient, individual or neighborhoods of IP addresses associated with the sender model for a sender are automatically determined.

FIG. 9 is a flowchart illustrating an embodiment of a process for determining a total risk measure of a contact. The process of FIG. 9 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. At least a portion of the process of FIG. 6 may be performed in one or more steps of the process of FIG. 2 (e.g., in 206).

At 902, a measure of trust associated with a message contact (e.g., sender of a message) is determined. In some embodiments, the measure of trust (e.g., numerical value) indicates a measure that the message contact is likely to send a message that is of value to a recipient/user. For example, a higher measure of trust indicates a higher likelihood that an authentic message sent by the contact will not include malicious and/or undesired content (e.g., message will not include a virus, malware, spam, impersonation attack, etc.). The measure of trust may be for a domain of the message contact that covers all messages of the same domain, a group of domains (e.g., domains of similar category), a group of contact addresses, or a specific contact address (e.g., email address). In some embodiments, the measure of trust is for a specific recipient/user. For example, the measure of trust indicates a measure of trust between the message contact and a specific recipient/user. In this example, the measure of trust may be determined based on a history of interaction between the message contact and the specific user.

In some embodiments, the measure of trust is determined at least by a portion of the process of FIG. 8. For example, the measure of trust is the specific measure of local reputation determined in 804. In another example, the measure of trust is the measure of global reputation included in the global sender model determined in 806. In another example, the measure of trust is determined based on a combination (e.g., average, sum, product, etc.) of the measure of global reputation and the specific measure of local reputation.

At 904, a measure of spoofing risk associated with the message contact is determined. For example, a likelihood that a spoofing attack could be technically carried out to spoof the message contact is determined. The measure of spoofing risk may be for a domain of the message contact that covers all messages of the same domain, a group of domains, a group of contact addresses, or a specific contact address (e.g., email address). The measure of spoofing risk may be a numeric score corresponding to the spoofing risk associated with the message contact.

An organization with a published and complete Domain-based Message Authentication, Reporting and Conformance (i.e., DMARC) policy has a low risk of being spoofed, as does each user account associated with such an organization. An organization that has a DMARC policy but which does not reject all messages that fail corresponds to a medium risk, as do users of such an organization. An organization without a DMARC policy or one that only involves monitoring and no rejection is associated with a high risk of being spoofed, as are users of such an organization. For example, if the message contact is associated with a DMARC policy that requires every message of the domain of the message contact to be signed with a digital signature, the measure of spoofing risk is assigned a low risk value. If the message contact is associated with a DMARC policy that enables but does not require every message of the domain of the message contact to be signed with a digital signature, the measure of spoofing risk is assigned a medium risk value. If the message contact is not associated with a DMARC policy, the measure of spoofing risk is assigned a high risk value. Thus, determining the measure of spoofing risk associated with the message contact may include assigning a score based on the DMARC or other message validation system policy associated with the message contact.

At 906, the measure of trust and the measure of spoofing risk are combined to determine a measure of total risk associated with the message contact. For example, the measure of trust and the measure of spoofing risk are multiplied together to determine the measure of total risk. In another example, the measure of trust and the measure of spoofing risk are averaged together to determine the measure of total risk. In another example, the measure of trust and the measure of spoofing risk are summed together to determine the measure of total risk. Thus, the measure of total risk is able to measure the threat of the message contact being impersonated, both in terms of the likely impact to the recipient (e.g., measure of trust provides a gauge on the likelihood the message recipient is likely to confuse the spoofed message as a message from a highly trusted and important contact) and the likelihood of technical feasibility (e.g., measure of spoofing risk provides a gauge on the measure of technical feasibility). In some embodiments, the measure of trust and the measure of spoofing risk may be each weighted before being combined. In some embodiments, the determined measure of total risk is stored in a data structure and associated with the message contact.

At 908, the measure of total risk is utilized to mitigate risk of messages received from the message contact. For example, when a message from the message contact is received, its measure of total risk for the sender of the message (e.g., message contact) is obtained and the measure of total risk is utilized to determine whether to filter and/or allow the message to be delivered. In an example, if a value of the measure of total risk is above a threshold value, the message is identified as a risk and the message is filtered to reduce its risk to the recipient.

FIG. 10 is a flowchart illustrating an embodiment of a process for assessing a received message for filtering. The process of FIG. 10 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. At least a portion of the process of FIG. 10 may be performed in 208, 210 and/or 212 of FIG. 2. At least a portion of the process of FIG. 10 may be performed in 306 of FIG. 3.

At 1002, information about a message is received. In some embodiments, the information is received at analysis server 102 of FIG. 1. The information may include information about a sender of the message. For example, the information includes email address header information including an email address and a display name of the sender of the message. In some embodiments, the information about the message includes contents of the message. Examples of the message include an electronic mail (i.e., email), an instant message, a chat message, and any other forms of electronic messages.

At 1004, it is determined whether a sender of the message exactly matches a trusted contact. For example, it is determined whether an address of the sender of the message (e.g., in a “From:” field of a message header) exactly matches an address of a trusted contact in a list of trusted contacts for the recipient of the message. In some embodiments, the trusted contacts include trusted contacts identified in 702 and/or 704 of FIG. 7. For example, a list of trusted contacts for the recipient of the message is obtained from storage and email addresses for each trusted contact of the list are compared with the email address of a sender of the message to identify whether there is an exact match.

If at 1004 it is determined that the sender of the message exactly matches the trusted contact, at 1006, it is determined to allow the message to be fully accessed by the recipient, if applicable. For example, because the sender of the message exactly matches the trusted contact, the message is allowed to be delivered (e.g., by sending the message to a mail delivery server and indicating that the message is allowed to be delivered) to the recipient user. In some embodiments, the message is allowed to be accessed by the recipient without message filtering that would otherwise be performed if the message was identified as potentially a part of an impersonation attack. In some embodiments, the message is further analyzed prior to allowing it to be fully accessed by the recipient. For example, the message may be analyzed to identify whether it includes a virus, a spam, or a malware. In some embodiments, the total risk score determined using 906 of FIG. 9 is utilized to determine whether to allow full access to the message. If a value of the measure of total risk of the sender is above a threshold value, the message is identified as a risk and the message is filtered as in 1010 to reduce its risk to the recipient.

If at 1004 it is determined that the sender of the message does not exactly match a trusted contact, at 1008, it is determined whether the sender of the message is similar to a trusted contact. For example, by determining whether the sender of the message is similar to a trusted contact, a likelihood of confusion by a recipient of the message that the message is from a trusted contact is determined. In some embodiments, the determining whether the sender of the message is similar to a trusted contact includes determining a measure of similarity between the sender of the message and each contact in a list of trusted contacts. In some embodiments, the list of trusted contacts includes trusted contacts identified in 702 and/or 704 of FIG. 7. For example, a list of trusted contacts for the recipient of the message (e.g., recipient identified in a “To:” field of the message) is obtained from storage.

In some embodiments, determining the measure of similarity includes determining a string similarity measure (e.g., string distance) using a string matching algorithm (e.g., Jaro-Winkler). For each trusted contact in a group of trusted contacts, string similarity measures may be determined between an address of the trusted contact and/or a display name of the trusted contact with an address of the sender of the message (e.g., string in a “From:” field of the message between “<” and “>” characters), a display name of the sender of the message (e.g., string in a “From:” field of the message prior to “<” character), and/or a subject of the message. For example, potentially six string similarity measures may be determined for each trusted contact email/display name combination (e.g., trusted contact address compared to sender address, trusted contact address compared to sender display name, trusted contact address compared to message subject, trusted contact display name compared to sender address, trusted contact display name compared to sender display name, and trusted contact display name compared to message subject).

Comparison of the subject to the message to the trusted contact address/display name is performed to detect masquerade attempts in which the actual sender uses deceptive display names and/or subject lines to trick recipients that emails are from a trusted sender. For example, the sender joe123@gmail.com may set his display name to appear as a subject line, and use a subject line that appears as a from email address. This attacker may use the display name “Please review this material and let me know your thoughts. Fwd: slide deck, June 15 version,” and the subject line “Bob Bigboss <bbigboss@bigbossventures.com>” to an email. The display name, being very long, will often hide the attacker's email address (e.g., which is displayed after the display name for a typical mail client, or not displayed at all for some mail clients). Many recipients will not realize that the subject line appears where the sender address is normally shown, and vice versa. Therefore, such recipients will believe that the email comes from Bob Bigboss <bbigboss@bigbossventures.com>, despite it being displayed in the subject field. In some embodiments, the subject of the message is only utilized to perform the determination of the measure of similarity if the display name of the sender of the email is longer than a threshold length.

In some embodiments, prior to performing the string comparison to determine the measure of similarity, element portions of the strings are sorted (e.g., alphabetically sorted) based on a predetermined order. The element portions of the string may be delimited by a space character or other special characters (e.g., comma, period, etc.). For example, strings “Bob Bigboss” and “Bigboss Bob” may be determined to be not similar in string distance despite them being a simple reversal of the order of the first/last names. Thus, the element portions “Bob” “Bigboss” in “Bob Bigboss” can be alphabetically sorted as “Bigboss Bob” prior to being used in the string comparison.

In some embodiments, determining the measure of similarity includes detecting use of substitution characters that mimic corresponding traditional alphabet/number characters in the sender address, sender display name, and/or subject of the message prior to comparison with identifiers of the trusted contact. For example, an attacker may use substitution characters to visually mimic a character in an identifier of a trusted contact (e.g., replace “o” with zero, replace with Cyrillic letter, replace with Greek letter, replace with symbol resembling a traditional letter/number, etc.). In some embodiments, if substitution characters are detected, various versions of the sender address, sender display name, and/or subject of the message that replace one or more different combinations of substitution characters with corresponding traditional characters are generated and utilized in the string comparisons with the identifiers of the trusted contacts.

In some embodiments, it is determined that the sender of the message is similar to a trusted contact if any measure of similarity (e.g., string distance measure) meets a threshold value. For example, if any string distance measure determined for a list of trusted contacts is greater than a threshold value, the message is identified as potentially an impersonation attack message and it is determined that the sender of the message is similar to the trusted contact.

In some embodiments, messages that are highly authentic (e.g., associated with a valid DMARC signature, correct SPF records, or both) but which come from domains that are associated with low control are to be subject to increased analysis. For example, a measure of control of the domain of the sender is determined. The measure of control measures how easy it is to create an account of the domain without having to pass a careful screening. For example, any user is able to create a Google Gmail account by setting the username to any available user name and setting the display name to any correctly formed display name. Thus, domains correspond to a very low degree of control by the domain owner. In contrast, the typical enterprise or government entity has high control over account creation, allowing only employees and affiliates to create accounts, where these have to satisfy some common criteria related to the real-life identity of the user. When the message is identified as having high authenticity but low domain control, then the display name is to be scrutinized (e.g., compared to trusted display names using string distance as previously discussed), whereas for domains with high control, the display name is not scrutinized (e.g., display name impersonation detection not performed).

If at 1008 it is determined that the sender of the message is similar to a trusted contact, at 1010, it is determined to filter the message, if applicable. Filtering the message may include modifying at least a portion of the message, affecting delivery of the message, providing a notification regarding the message, and/or performing additional analysis on the message. An example of filtering the message is performing a security action.

In some embodiments, modifying at least a portion of the message includes modifying (e.g., replacing, adding or removing/deleting) data included in a data field (of the message) that includes an identification of the sender of the message. For example, the data field that is identified to include the identification of the sender is selected from a plurality of data fields of a message header and contents of the selected data field is modified. Examples of the identification of the sender includes a display name, an email address (e.g., “from” email address), a reply-to address, a phone number, an image, a content link, or any other identifier/identification of the sender specified in the message. The data field of the message may be included in a header of the message. For example, the data field of the message is a “From:” field, a “Reply-To:” field or a “Return-Path:” field of an email message header. In some embodiments, the message is a SMS message or a MMS message and a phone number of the sender in the data field is modified. For example, the sender phone number is removed or changed to a different number. In some embodiments, modifying data included in the data field includes modifying a location identifier of content associated with the sender of the message. For example, a URL to an image of the sender is modified to reference a different image (e.g., replacement image includes a warning message).

One example filtering is quarantining the message and sending an automated message to the email address in the “From:” field of the message regarding the quarantine. The automated email may require the recipient to click on a hyperlink that is unique and not predictable to an attacker. If the system determines that such a link was clicked on (e.g., by receiving a corresponding GET request), then the email may be taken out of quarantine and delivered.

Another example filtering is sending an automated email in response to the received message (e.g., whether to the address in the from field or a potential address in the ‘reply-to’ field or both), requiring the recipient to click on a hyperlink where he or she has to enter a code obtained from an authenticator module, such as a SecurID token or Google authenticator app. If the entered code is correct, the message is taken out of quarantine and delivered.

In an alternative embodiment, the received message is not quarantined but delivered to its recipient after being modified. For example, a warning message is added to one or more header fields and/or the body of the message (e.g., add warning message to subject of message, add warning message to modify sender address, etc.). In some embodiments, the received message is modified to change, remove, and/or censure at least a portion of the message (e.g., remove attachment, remove hyperlinks, remove/modify reply to address, etc.).

If at 1008 it is determined that the sender of the message is not similar to a trusted contact, the process returns to 1006. In an alternative embodiment, if at 1008, it is determined that the sender of the message is not similar to a trusted contact, the message is identified for filtering if the recipient of the message has never sent a message to the sender of the message. In some embodiments, the total risk score determined using 906 of FIG. 9 is utilized to determine whether to allow full access to the message. If a value of the measure of total risk of the sender is above a threshold value, the message is identified as a risk and the message is filtered in 1010 to reduce its risk to the recipient.

FIG. 11A is a flowchart illustrating an embodiment of a process for modifying a display name of a sender of the message. The process of FIG. 11A may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 11A is performed in filtering the message in 1006 and/or 1010 of FIG. 10. At least a portion of the process of FIG. 11A may be performed in 212 of FIG. 2. For example, 1104 of FIG. 11A is performed in 212 FIG. 2 as a selected security action to perform.

At 1102, a message is identified as suspicious. For example, it is identified in 708 that a sender of the message is similar to a trusted contact based on a measure of similarity of a sender of the contact with a trusted contact of a particular identified recipient of the message. An example of a trusted contact is a contact that has been previously identified by the recipient (e.g., included in an address book of the recipient) or is a contact with whom the recipient has had at least a minimum amount of positive engagement (e.g., having exchanged emails at least twice, and at least two weeks apart) without a recent “negative engagement” (e.g., recipient has not recently marked a message from the contact as spam). In some embodiments, the message is identified as suspicious if a sender of the message has not previously sent a message to a recipient identified in the message. In some embodiments, the message is identified as suspicious if a recipient identified in the message has not previously sent a message to the sender of the message. In some embodiments, the message is identified as suspicious if a spam detector, a virus detector, and/or a malware detector has detected that the message includes a spam, virus, or malware.

At 1104, a display name of a sender of the message is modified prior to allowing the message to be accessed by a recipient user of the message. For example, the display name of the sender is rewritten to include a warning regarding its risk. In one example, consider an email with display name “Pat Peterson,” but not associated with an email address of a Pat Peterson that is considered trusted. This may cause the system to determine that the email is risky, and modify the display name to “Warning! Unknown user with the name ‘Pat Peterson.’” Conversely, if an email is determined to come from a trusted sender, the system may modify the display name by adding symbol(s) (e.g., icon of a star) identifying its risk. In some embodiments, display names of senders in messages identified as not suspicious are normalized, which may involve removing anything looking like a trust indicator (e.g., such as an icon of a star) from the display name, or adding another symbol to the display name. Alternatively, a message from a user that is neither considered risky nor trusted could have its display name removed and replaced by only the message address of the sender (e.g., only email address without display name), drawing the attention of the recipient to the fact that this is a user with whom the system has not determined there to be a trust relationship. This may be particularly beneficial to users who use mail clients that do not display the full email address, but only show the display name.

In some embodiments, in addition to or instead of the display name, an email address of the sender is also modified. For example, the entire “From:” field of a header of the message that includes both the display name and an email address is modified with a warning.

FIG. 11B is a flowchart illustrating an embodiment of a process for modifying a reply address of a sender of the message. The process of FIG. 11B may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 11B is performed in filtering the message in 1006 and/or 1010 of FIG. 10. At least a portion of the process of FIG. 11B may be performed in 212 of FIG. 2. For example, 1104 of FIG. 11B is performed in 212 FIG. 2 as a selected security action to perform.

At 1112, a message is identified as suspicious. For example, it is identified in 508 that a sender of the message is similar to a trusted contact based on a measure of similarity of a sender of the contact with a trusted contact of a particular identified recipient of the message. An example of a trusted contact is a contact that has been previously identified by the recipient (e.g., included in an address book of the recipient) or is a contact with whom the recipient has had at least a minimum amount of positive engagement (e.g., having exchanged emails at least twice, and at least two weeks apart) without a recent “negative engagement” (e.g., recipient has not recently marked a message from the contact as spam). In some embodiments, the message is identified as suspicious if a sender of the message has not previously sent a message to a recipient identified in the message. In some embodiments, the message is identified as suspicious if a recipient identified in the message has not previously sent a message to the sender of the message. In some embodiments, the message is identified as suspicious if a spam detector, a virus detector, and/or a malware detector has detected that the message includes a spam, virus, or malware.

At 1114, a reply address of a sender of the message is modified prior to allowing the message to be accessed by a recipient user of the message. In the event where the message does not have a reply address, a “Reply-To:” field is automatically added to the message. This can be a real email address, different from the “from” address of the received message, and act as a quarantine for outgoing responses to high-risk emails. In such a quarantine, the recipient system can carefully verify (e.g., whether automatically or using manual effort, or a combination thereof) that the outgoing message is safe before it is automatically sent to the “from” address of the received email.

In some embodiments, an address in a “Reply-To:” field of an email header is replaced or inserted with a warning message (e.g., not a valid reply address) regarding the suspicious risk of the message. In one example, the “reply to” address of a message is replaced/created as “Warning! This is a user you have never sent an email to—If you want to proceed, remove this text from the recipient field of the email you want to send, and add the address of the recipient you wish to send it to.” This reply address will cause a pop-up on some message clients with the text above (e.g., and an error stating that it is not a valid email address) or may cause on some message clients to bounce the email.

FIG. 12 is a flowchart illustrating an embodiment of a process for securing a message. The process of FIG. 12 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 12 is performed in filtering the message in 1006 and/or 1010 of FIG. 10. At least a portion of the process of FIG. 12 may be performed in 208, 210 and/or 212 of the process of FIG. 2. At least a portion of the process of FIG. 12 may be performed in 306 of the process of FIG. 3.

At 1202, a first risk analysis of the message is performed. For example, it is determined whether the message is suspicious. In some embodiments, steps 1004 and/or 1006 of FIG. 10 is performed in 1202. For example, it is determined whether the sender of the message is similar to a trusted contact. By determining whether the sender of the message is similar to a trusted contact, a likelihood of confusion by a recipient of the message that the message is from a trusted contact may be determined. In some embodiments, the determining whether the sender of the message is similar to a trusted contact includes determining a measure of similarity between the sender of the message and each contact in a list of trusted contacts. In some embodiments, the list of trusted contacts includes trusted contacts identified in 702 and/or 704 of FIG. 7. For example, a list of trusted contacts for the recipient of the message (e.g., recipient identified in a “To:” field of the message) is obtained from storage.

In some embodiments, determining the measure of similarity includes determining a string similarity measure (e.g., string distance) using a string matching algorithm (e.g., Jaro-Winkler). For each trusted contact in a group of trusted contacts, string similarity measures may be determined between an address of the trusted contact and/or a display name of the trusted contact with an address of the sender of the message (e.g., string in a “From:” field of the message between “<” and “>” characters), a display name of the sender of the message (e.g., string in a “From:” field of the message prior to “<” character), and/or a subject of the message. For example, potentially six string similarity measures may be determined for each trusted contact email/display name combination (e.g., trusted contact address compared to sender address, trusted contact address compared to sender display name, trusted contact address compared to message subject, trusted contact display name compared to sender address, trusted contact display name compared to sender display name, and trusted contact display name compared to message subject).

Comparison of the subject to the message to the trusted contact address/display name is performed to detect masquerade attempts in which the actual sender uses deceptive display names and/or subject lines to trick recipients that emails are from a trusted sender. For example, the sender joe123@gmail.com may set his display name to appear as a subject line, and use a subject line that appears as a from email address. This attacker may use the display name “Please review this material and let me know your thoughts. Fwd: slide deck, June 15 version,” and the subject line “Bob Bigboss <bbigboss@bigbossventures.com>” to an email. The display name, being very long, will often hide the attacker's email address (e.g., which is displayed after the display name for a typical mail client, or not displayed at all for some mail clients). Many recipients will not realize that the subject line appears where the sender address is normally shown, and vice versa. Therefore, such recipients will believe that the email comes from Bob Bigboss <bbigboss@bigbossventures.com>, despite it being displayed in the subject field. In some embodiments, the subject of the message is only utilized to perform the determination of the measure of similarity if the display name of the sender of the email is longer than a threshold length.

In some embodiments, prior to performing the string comparison to determine the measure of similarity, element portions of the strings are sorted (e.g., alphabetically sorted) based on a predetermined order. The element portions of the string may be delimited by a space character or other special characters (e.g., comma, period, etc.). For example, strings “Bob Bigboss” and “Bigboss Bob” may be determined to be not similar in string distance despite them being a simple reversal of the order of the first/last names. Thus, the element portions “Bob” “Bigboss” in “Bob Bigboss” can be alphabetically sorted as “Bigboss Bob” prior to being used in the string comparison.

In some embodiments, determining the measure of similarity includes detecting use of substitution characters that mimic corresponding traditional alphabet/number characters in the sender address, sender display name, and/or subject of the message prior to comparison with identifiers of the trusted contact. For example, an attacker may use substitution characters to visually mimic a character in an identifier of a trusted contact (e.g., replace “o” with zero, replace with Cyrillic letter, replace with Greek letter, replace with symbol resembling a traditional letter/number, etc.). In some embodiments, if substitution characters are detected, various versions of the sender address, sender display name, and/or subject of the message that replace one or more different combinations of substitution characters with corresponding traditional characters are generated and utilized in the string comparisons with the identifiers of the trusted contacts.

At 1204, it is determined whether the first risk analysis results in a determination that the message meets a first criteria. For example, it is determined whether the message meets a criteria to be deemed suspicious. In some embodiments, message meets the first criteria if a measure of similarity (e.g., string distance measure) meets a threshold value. For example, if any string distance measure determined for a list of trusted contacts (e.g., determined in 1202) is greater than a threshold value, the message is identified as potentially an impersonation attack message and it is determined that the message meets the first criteria.

In some embodiments, the message meets the first criteria if a sender of the message has not previously sent a message to a recipient identified in the message. In some embodiments, the message meets the first criteria if a recipient identified in the message has not previously sent a message to the sender of the message. In some embodiments, the message meets the first criteria if a spam detector, a virus detector, and/or a malware detector has detected that the message includes a spam, virus or malware.

In some embodiments, a total risk score determined using 906 of FIG. 9 is utilized to determine whether message meets the first criteria. For example, if a value of the measure of total risk of the sender is above a threshold value, the message is identified as meeting the first criteria.

If at 1204 it is determined that the message does not meet the first criteria, at 1206 it is determined to allow the message to be fully accessed by the recipient, if applicable. For example, because the sender of the message exactly matches the trusted contact or sender of the message is not similar to any trusted contact, the message is allowed to be delivered (e.g., by sending the message to a mail delivery server and indicating that the message is allowed to be delivered) to the recipient user. In some embodiments, the message is allowed to be accessed by the recipient without message filtering that would otherwise be performed if the message was identified as potentially a part of an impersonation attack. In some embodiments, the message is further analyzed prior to allowing it to be fully accessed by the recipient. For example, the message may be analyzed to identify whether it includes a virus, a spam, or a malware.

If at 1204 it is determined that the message does meet the first criteria, at 1208 the message is modified prior to sending a modified version of the message to a specified recipient of the message. For example, a warning message is added to one or more header fields and/or the body of the message (e.g., add warning message to subject of message, add warning message to modify sender address, etc.). The first risk analysis is performed before sending the modified version of the message. Sending the modified version may include delivering the modified version to the specified recipient (e.g., deliver to a mailbox of the specified recipient on server 106 of FIG. 1).

In some embodiments, modifying at least a portion of the message includes modifying (e.g., replacing, adding or removing/deleting) data included in a data field of the message that includes an identification of the sender of the message (e.g., modify an identifier of the sender). Examples of the identification of the sender includes a display name, an email address (e.g., “from” email address), a reply-to address, or any other identifier/identification of the sender specified in the message. The data field of the message may be included a header of the message. For example, the data field that includes the identification of the sender is a “From:” field, a “Reply-To:” field or a “Return-Path:” field of an email message header. The contents of the data field may be modified to provide a warning to a recipient, prevent a potential confusion of an identity of the sender of the message by the recipient of the message and/or prevent the recipient from responding to the message or sending a message to the sender of the message.

In some embodiments, the received message is modified to change, remove, and/or censure at least a portion of a content of the message (e.g., remove attachment, remove hyperlinks, remove a portion of the message content beyond a selected beginning body portion, etc.). For example, a message body section of an email message is modified to censure at least a portion of a content of the email message. In some embodiments, modifying the message includes inserting an inquiry associated with a verification of the sender of the message, wherein the inquiry requests a user response.

At 1210, a second risk analysis of the message is performed and it is determined whether the second risk analysis results in a determination that the message meets the second criteria. In some embodiments, the modified version of the message is sent to the specified recipient of the message (e.g., sent to a mailbox of the specified recipient on server 106 of FIG. 1) prior to a conclusion of the second risk analysis. For example, the modified version of the message is sent and delivered to a mailbox of the specified recipient of the message prior to the conclusion of the second risk analysis. In some embodiments, the second risk analysis concludes upon determination of whether the second risk analysis results in a determination that the message meets the second criteria.

In some embodiments in 1210, any of a plurality of different types of verification/security actions may be performed to further analyze and verify the message (e.g., verify an identity of the sender of the message). For example, it is determined whether a classification of the sender of the message requested and received from the specified recipient of the message matches an automatically identified classification of the sender, and it is determined the message meets the second criteria if a user response to the requested classification matches the automatically identified classification (e.g., see the process of FIG. 13A). In another example, an automatically generated reply message to a received message is sent the sender of the received message requesting verification of an identity of the sender of the message and it is determined the message meets the second criteria if the sender has provided a valid identity in response to the reply message (e.g., see the process of FIG. 14A).

If at 1210 it is determined that the second risk analysis results in a determination that the message meets the second criteria, at 1212, content of the message that was previously prevented from being accessed by the specified recipient is provided to the specified recipient of the message. The second risk analysis may be a more in-depth and/or resource intensive analysis of the message reserved for messages that fail the first risk analysis. For example, an original version of the message prior to the modification in 1208 is sent to the specified recipient of the message. This original version may be a duplicate message in addition to the initially provided modified version of the message (e.g., original version delivered as a new message to the specified recipient) or this original version may be a replacement message of the initially provided modified version of the message (e.g., initially provided modified message is deleted from mailbox of the specified recipient of the message and replaced with the original version). In some embodiments, the content of the message that was previously prevented from being accessed is included a new message sent to the specified recipient of the message.

If at 1210 it is determined that the second risk analysis results in a determination that the message does not meet the second criteria, at 1214 content of the message that was prevented from being accessed by the specified recipient remains not accessible by the specified recipient. In some embodiments, the message is deleted or quarantined. For example, after a period of time of message not meeting the second criteria and/or if it is determined during the second risk analysis that the message is potentially hazardous to a specified recipient of the message, the message is automatically deleted or quarantined to prevent access by the specified recipient to the entire message. In some embodiments, a notification is provided to an administrator. For example, the administrator is provided a warning about the message failing the second risk analysis.

FIG. 13A is a flowchart illustrating an embodiment of a process for detecting misrecognition of a sender of a message by a recipient of the message. The process of FIG. 13A may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 13A is performed in one or more steps of the process of FIG. 2 and/or one or more steps of the process of FIG. 3. In some embodiments, at least a portion of the process of FIG. 13A is performed in one or more steps of the process of FIG. 12. For example, at least a portion of the process of FIG. 13A is performed in performing the first and/or second risk analysis of the process of FIG. 12.

At 1302, a message is identified as suspicious. For example, it is identified in 1008 that a sender of the message is similar to a trusted contact based on a measure of similarity of a sender of the contact with a trusted contact of a particular identified recipient of the message. An example of a trusted contact is a contact that has been previously identified by the recipient (e.g., included in an address book of the recipient) or is a contact with whom the recipient has had at least a minimum amount of positive engagement (e.g., having exchanged emails at least twice, and at least two weeks apart) without a recent “negative engagement” (e.g., recipient has not recently marked a message from the contact as spam). In some embodiments, the message is identified as suspicious if a sender of the message has not previously sent a message to a recipient identified in the message. In some embodiments, the message is identified as suspicious if a recipient identified in the message has not previously sent a message to the sender of the message. In some embodiments, the message is identified as suspicious if a spam detector, a virus detector, and/or a malware detector has detected that the message includes a spam, virus, or malware.

In some embodiments, by basing the whether the message is suspicious on knowledge about trusted contacts of the specified recipient of the message, a message is only classified as suspicious and risky based on the likelihood that the recipient will confuse the sender with a trusted contact specific to the recipient. Otherwise, the risk may be marginal if the recipient does not know or trust the sender of the message since the sender is likely not impersonating someone that the recipient already knows. Thus a message is particularly dangerous if it is not from a trusted party, but the recipient thinks it is. In contrast, if the recipient knows that an email from an untrusted party is from an untrusted party, he or she is not as much at risk as he or she is not likely to feel compelled to comply with the requests or instructions in the message

At 1304, the sender of the message is automatically classified. In some embodiments, classifying the sender includes identifying a classification/category of the sender. For example, a relationship classification/category of the sender with respect to the specified recipient of the message is automatically determined based on a property of an identifier of the sender (e.g., domain), previous message history between the sender and recipient, and/or other list or database (e.g., database of business contacts of the recipient). Examples of the classification of the sender include a college, a business partner, a friend/acquaintance, or unknown.

For example, if the sender is associated with the same domain as the recipient, then the sender is classified as a colleague classification. This classification may also be used if the email address of the sender is a known personal email address of a user with a colleague message address. This personal address may be determined using manual registration of such addresses by the account owner, as well as by identification of message traffic indicative of a work account being related to a personal account, such as having closely related display names and extensive forwarding of material from one account to the other. If the sender is a service provider or a client of an entity associated with the recipient, then the sender is classified as a business partner classification. This type of classification may be identified by detection of a sufficient volume of message traffic over time between the sender and the recipient, or from a database/list of recorded business contacts/relationships. In some embodiments, the sender is classified as a friend/acquaintance classification for a given sufficient volume of message traffic over time between the sender and the recipient but cannot be classified as a colleague or a business partner. If the sender cannot be classified as a colleague, business partner, or friend/acquaintance, the sender is classified as known/stranger.

In some embodiments, the sender of the message is automatically classified based on a list of trusted contacts tracked for the recipient (e.g., tracked using the process of FIG. 7). For example, a trusted contact in the stored list of trusted contacts may be associated with a classification identifier provided by a user and/or automatically determined. An identifier of the sender of the email may be utilized to locate a corresponding entry in the stored list of trusted contacts to identify its classification.

At 1306, a manual classification of the sender of the message is requested from the specified recipient of the message. For example, before the message is fully provided to the recipient, a classification of the sender of the message is requested from the user in order to verify the recipient's believed classification of the sender as compared to the automatically determined classification. In some embodiments, the message is modified to change, remove, or obfuscate (e.g., remove attachment, remove hyperlinks, hide phone numbers, only retain a beginning portion of contents of the message and hide ending portion of the message, etc.) at least a portion of the message to prevent the recipient from having full access to the message prior to verification of the recipient's knowledge about the sender of the message. In some embodiments, the message is modified to include selection choices corresponding to the classification options (e.g., different selectable HTML hyperlinks included in the message and selection of a particular hyperlink option sends a request to the unique address of the hyperlink that can be detected by a server to identify which classification option was selected based on the unique address of the request corresponding to the selection option). In some embodiments, if a recipient replies to the message to the sender prior to providing the classification, the reply message is held/quarantined at a mail server until it is determined that it is safe to send the message. Another classification option is to identify the sender as a spammer or malware attacker. Selection of this classification may result in the removal of the email and blocking of future messages from the sender.

FIG. 13B shows an example of a modified message requesting manual classification of the sender of the message. Message 1320 includes contents of the original message 1322 as well as added selected choices 1324 that requests the recipient to classify the sender of the message.

Returning to FIG. 13A, at 1308, based on the response to the manual classification request, a security action is performed, if applicable. In some embodiments, if the manual classification response does not match the automatically determined classification, a security action is performed to warn the recipient about the mismatch and/or prevent the user from fully accessing or replying to the message. For example, the message may be deleted and replaced with a version of the message that has been modified (e.g., subject, sender display name, sender address, reply to address, and/or content of the message removed or modified with a warning indicator). In another example, the message may be deleted and quarantined for additional review by the recipient and/or an administrator. In some embodiments, in the event the sender of the message was correctly classified manually by the recipient and the automatic classification was incorrect, an opportunity is provided to add the sender of the message to the list of trusted contacts for the recipient and/or add an identifier of the sender (e.g., email address and display name) to an existing contact (e.g., as a secondary email address) in the list of trusted contacts. The sender may be added to the list of trusted contacts by the recipient, an administrator, or by the sender (e.g., by requiring the user to prove access to an account of an existing trusted contact).

FIG. 14A is a flowchart illustrating an embodiment of a process for requiring a sender of a message to verify an identity of the sender. The process of FIG. 14A may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 14A is performed in filtering the message in 1006 and/or 1010 of FIG. 10. In some embodiments, at least a portion of the process of FIG. 14A is performed in one or more steps of the process of FIG. 12. For example, at least a portion of the process of FIG. 14A is performed in performing the first and/or second risk analysis of the process of FIG. 12. At least a portion of the process of FIG. 14A may be performed in one or more steps of the processes of FIG. 2 and/or FIG. 3. For example, it is performed as a security action in 212 of FIG. 2.

At 1402, a message is identified as suspicious. For example, it is identified in 1008 that a sender of the message is similar to a trusted contact based on a measure of similarity of a sender of the contact with a trusted contact of a particular identified recipient of the message. An example of a trusted contact is a contact that has been previously identified by the recipient (e.g., included in an address book of the recipient) or is a contact with whom the recipient has had at least a minimum amount of positive engagement (e.g., having exchanged emails at least twice, and at least two weeks apart) without a recent “negative engagement” (e.g., recipient has not recently marked a message from the contact as spam). In some embodiments, the message is identified as suspicious if a sender of the message has not previously sent a message to a recipient identified in the message. In some embodiments, the message is identified as suspicious if a recipient identified in the message has not previously sent a message to the sender of the message. In some embodiments, the message is identified as suspicious if a spam detector, a virus detector, and/or a malware detector has detected that the message includes a spam, virus, or malware.

At 1404, the message is prevented from being fully accessible by the specified recipient of the message. For example, at least a portion of the message is modified or removed. In another example, the message is quarantined and not delivered to the recipient.

At 1406, an automatic reply to the message is sent requesting verification of an identity of the sender of the message.

This verification attempt may be utilized to identify good emails from accounts that are not trusted, but which are nevertheless associated with trusted persons. For example, if one executive's name is John Adams, and another executive receives an email from a John Adams—but not the same account as to which she has a trust relationship—then this is a potentially deceptive email—but also an email that is potentially from another account (such as the personal account) of the trusted person. The reply message includes the message “Your email triggered an alert. Have you sent emails to the recipient using another email address? If so, please click here. Otherwise, click here.” The first hyperlink (associated with the first occurrence of “here”) would lead to a website where the visitor enters his or her “other” email address (e.g., trusted contact address), and a verification email is sent to that address. That verification email contains a challenge value, such as a secret string of alphanumeric characters, that the recipient has to copy and paste into the site where the email address was entered. The second hyperlink takes the user to a site where he or she is offered to solve a CAPTCHA, associate a phone number (e.g., by sending an SMS challenge value to the phone number), or perform another action that attackers are typically unwilling to perform.

FIG. 14B shows an example of an automatic reply message requesting verification of an identity of the sender of the message. Message 1420 informs the user that the sender's message has been quarantined and will not be delivered unless identity of the sender is established. Message 1420 instructs the sender to forward the automatic reply message to a second message account (e.g., account of trusted contact) that the sender has previously used to communicate with the recipient and using the second account, forward again the forwarded message to a verification system for verification of the possession of the second account.

Returning to FIG. 14A, the automatic reply to the message may be utilized to detect spoofing attempts. For example, the automatic reply is sent to a system of the sender to request verification that the sender identified in the message actually sent the message. For example, at least two installations of message security systems, where a first installation is associated with a first entity and a second installation is associated with a second entity, the second system identifies receipt of an email originating with a sender associated with the first system. The second system then sends an inquiry to the first system, where the inquiry contains information related to the email. The first system determines whether the email associated with the inquiry was sent from the indicated apparent sender, and generates a response to the second system. If the second system receives a negative response, then the email is not delivered to the intended recipient, or if it has already been delivered, then it is either removed from the inbox or a notification is sent to the recipient, warning the recipient of the email. In addition, a notification may be sent to an administrator associated with the second system. In some embodiments, a notification is also sent to an admin associated with the first system. If, on the other hand, the response to the inquiry is a positive response, then the email is delivered to the recipient. If an email has more than one recipient within the second system, then it is delivered to each such recipient. Here, the first and second systems may access emails in different ways. For example, the first system may correspond to an outbound collector of emails and the second system to an OAUTH access of one or more email boxes. The first system may also be an appliance controlling or relaying emails, or be an OAUTH-enabled method to access the various mailboxes (including the sent folder) of one or more users. In some embodiments, all communications between the two security systems are encrypted and authenticated.

At 1408, if the sender has provided a valid identity in response to the automatic reply in 1406, the message is allowed to be fully accessible by the specified recipient. For example, if the user has provided an established position of an alternate trusted account that can be trusted (e.g., account of trusted contact) or has otherwise provided or proved an identity of the sender, the message is allowed to be fully accessible. Making the message fully accessible may include allowing the message to be delivered and accessed by the recipient with all of its original content. This may also cause the sender (e.g., sender email address) to be whitelisted or associated with an increased trust value (e.g., add to trusted contacts). In some embodiments, the provided full access message includes a warning, bringing to the attention of the recipient that this is an email from a potential stranger.

If the sender does not provide a valid identity in response to the automatic reply in 906, the message may remain unable to be fully accessible by the specified recipient of the message. In some embodiments, the message is indicated to an administrator and/or deleted.

In some embodiments, if the reply to the address of the message is identified or unknown to be unattended (e.g., not monitored by a recipient user), the message is provided to the recipient in a modified form and/or with a warning.

FIG. 15 is a flowchart illustrating an embodiment of a process for determining a change in historical reputation of a sender of a message. The process of FIG. 15 may be at least in part implemented on analysis server 102, gateway 110, and/or message server 106 of FIG. 1. In some embodiments, at least a portion of the process of FIG. 15 is performed in filtering the message in 1006 and/or 1010 of FIG. 10. In some embodiments, at least a portion of the process of FIG. 15 is performed in one or more steps of the process of FIG. 12. For example, at least a portion of the process of FIG. 15 is performed in performing the first and/or second risk analysis of the process of FIG. 12. At least a portion of the process of FIG. 15 may be performed in one or more steps of the process of FIG. 2.

At 1502, a long term measure of reputation associated with a sender of a message is determined. The long term measure of reputation may be for a specific address and/or domain of the sender. For example, long term measure of reputation is determined based on the amount of historical message traffic between the sender (or network domain of sender) and the recipient (or network domain of recipient). Large amounts of traffic may correspond to a high long term measure of reputation, unless there is a sufficient portion of the traffic that is flagged as undesirable (e.g., being spam, including virus/malware, etc.). Additionally, a long time period of traffic may correspond to a high long term measure of reputation, unless a sufficient portion is flagged as being undesirable. To the extent that it is available, a large open rate (e.g., the recipient opened the message) or a high response rate (e.g., whether replying to message or selecting links in message) also may correspond to a high long term measure of reputation, unless a sufficient portion is flagged as being undesirable. An example of a “sufficient” portion may mean that a fixed portion of the traffic, such as 2%. The long term measure of reputation may capture historical message traffic within several recent months. In some embodiments, when determining a long term measure of reputation for a domain, individual long term measures of reputation for accounts of the domain may be aggregated to determine the long term measure of reputation for the domain.

At 1504, a recent short term measure of reputation associated with the sender of the message is determined. The recent short term measure of reputation may be for a specific address and/or domain of the sender. The recent short term measure of reputation is determined using the same/similar factors as the long term measure of reputation but within a smaller time window. For example, the recent short term measure of reputation may capture historical message traffic within a day.

The recent short term measure of reputation may be low if a large portion (e.g., 5% of traffic within the time window) of message traffic between the sender (or network domain of sender) and the recipient (or network domain of recipient) is associated with complaints, spam detection, the detection of unwanted URLs, or unwanted attachments. An unwanted URL/attachment is one that is judged likely to be associated with risk, e.g., using a blacklist or an anti-virus scan. Complaints may correspond to users reporting an email as spam or phish, or placing it in a spam folder. Complaints can either be local (only a very small number of users, such as less than 1% of recipients, report the message) or pervasive (a greater portion of users report the email, e.g., more than 1%).

At 1506, changes in the recent short term measure with respect to the long term measure of reputation are detected and utilized to determine whether and/or how to filter the message. For example, if the sender is associated with a low (bad) long term measure of reputation (e.g., below a threshold) and a low (bad) recent short term measure (e.g., below another threshold), the message from the sender is identified as suspicious and filtered (e.g., blocked and/or modified with a warning). In some embodiments, if the sender is associated with a high (good) long term measure of reputation (e.g., above a first threshold) but low (bad) recent short term measure (e.g., below a second threshold), the message from the sender is identified as likely taken over by a malicious attacker. For example, the message may be filtered (e.g., blocked and/or modified with a warning), may require manual review by an administrator, or require an action by the sender (e.g., require sender to respond to a challenge) prior allowing delivery of the message (e.g., modified message with warning).

In some embodiments, let Ri be a reputation associated with a receiver, and for concreteness, assume that this corresponds to a numeric value between 0 and 1, inclusive. Let Bi be a recent short term measure of reputation associated with an email E received by the same recipient. For concreteness, this is also a value between 0 and 1, inclusive. Here, i is a number corresponding to the recipient. Further, let G be the long term measure of reputation associated with the email E, and may be a number between 0 and 100, for example. The value SUM(Bi*Ri)/G is computed, where SUM corresponds to adding the entries associated with all recipients of E. If SUM(Bi*Ri)/G>t1, where t1 is a first threshold, then a first action is taken. This action may be to block emails from the sender of E, remove E from the inbox of all recipients, and determine whether any of the users who appear to have opened E have had their computers corrupted or credentials stolen. This can be determined by asking the user, by scanning the user's machine, or by reviewing logs of incoming/outgoing traffic to the user's machine, where this review is preferably automated. Further, the action may involve increasing Ri by 10% for each user who filed a response Bi indicative of a threat. If SUM(Bi*Ri)/G<t2, where t2<t1 then the reputation Ri of each user who filed a response Bi indicative of a threat is decreased, e.g., by 5%. If the number of users filing a response Bi is indicative of a threat within a time T of receipt of the email E, then the email is considered to be associated with pervasive badness, otherwise local badness.

Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive. 

What is claimed is:
 1. A method of detecting security risk, comprising: receiving information about an electronic message that is from a sender for an intended recipient; determining whether the sender of the electronic message has an established relationship with the intended recipient, wherein the established relationship is based at least in part on previous email messages between the sender and the intended recipient; in response to the determination that the sender of the electronic message has the established relationship with the intended recipient, using a processor to analyze the electronic message based at least in part on previously observed communications between the sender and the intended recipient to determine a security risk of the electronic message for the intended recipient using a statistically analyzed result identifying a likelihood of existence of a cluster of two or more email header items by tracking and matching combinations of corresponding email header items from email headers of previous email messages sent from the sender, wherein the cluster of the combination of two or more of the email header items include a mail user agent (MUA) metadata item, a time zone, an IP address, X-header metadata information, or an identification of a supported character set; and based on the determined security risk of the electronic message, performing a security action, if applicable.
 2. The method of claim 1, wherein determining whether the sender of the electronic message has the established relationship with the intended recipient includes determining whether the sender has previously sent the intended recipient a threshold number of messages.
 3. The method of claim 1, wherein determining whether the sender of the electronic message has the established relationship with the intended recipient includes determining whether the sender had been in communication with the intended recipient for at least a threshold amount of time.
 4. The method of claim 1, wherein determining whether the sender of the electronic message has the established relationship with the intended recipient includes obtaining information about the sender or user relationships of the intended recipient from a service that tracks social network relationship data.
 5. The method of claim 1, wherein determining whether the sender of the electronic message has the established relationship with the intended recipient includes determining a trust score associated with the sender with respect to the intended recipient of the message.
 6. The method of claim 1, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes using profiles of body content included in previous messages received from the sender.
 7. The method of claim 1, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes using profiles of header content included in the previous messages received from the sender.
 8. The method of claim 1, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes determining a plurality of risk component scores and combining the plurality of risk component scores to determine an overall risk score of the security risk of the electronic message for the intended recipient.
 9. The method of claim 1, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes determining a risk component score based on whether the message includes a keyword and whether the keyword or a related keyword was included in a previous message from the sender.
 10. The method of claim 1, wherein determining the security risk of the electronic message for the intended recipient includes computing a Bayesian probability score of a particular feature of message being consistent with past observations of previous messages from the sender.
 11. The method of claim 1, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes determining a probability a plurality of aspects of the electronic message have been exhibited together in previous messages from the sender for the intended recipient.
 12. The method of claim 1, wherein performing the security action includes selecting based on the determined security risk of the electronic message, which security action option to perform among a plurality of security action options.
 13. The method of claim 1, wherein performing the security action includes determining that a risk score associated with the security risk of the electronic message is below a threshold value and allowing the intended recipient to fully access the electronic message by delivering the electronic message to the intended recipient.
 14. The method of claim 1, wherein performing the security action includes determining that a risk score associated with the security risk of the electronic message is above a threshold value and modifying the electronic message prior to allowing the intended recipient to access a modified version of the electronic message.
 15. The method of claim 1, wherein performing the security action includes performing one or more of the following: sending a verification challenge to an alternative contact of the sender; performing additional analysis of the electronic message; quarantining the electronic message; blocking the electronic message; executing an executable included in the electronic message in a sandbox or a virtual machine; adding a warning to the electronic message; and moving the electronic message to a different folder.
 16. A system of detecting security risk, comprising: a hardware processor configured to: receive information about an electronic message that is from a sender for an intended recipient; determine whether the sender of the electronic message has an established relationship with the intended recipient, wherein the established relationship is based at least in part on previous email messages between the sender and the intended recipient; in response to the determination that the sender of the electronic message has the established relationship with the intended recipient, analyze the electronic message based at least in part on previously observed communications between the sender and the intended recipient to determine a security risk of the electronic message for the intended recipient using a statistically analyzed result identifying a likelihood of existence of a cluster of two or more email header items by tracking and matching combinations of corresponding email header items from email headers of previous email messages sent from the sender, wherein the cluster of the combination of two or more of the email header items include a mail user agent (MUA) metadata item, a time zone, an IP address, X-header metadata information, or an identification of a supported character set; and based on the determined security risk of the electronic message, perform a security action, if applicable; and a memory coupled to the processor and configured to provide the processor with instructions.
 17. The system of claim 16, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes using profiles of body content included in previous messages received from the sender.
 18. The system of claim 16, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes using profiles of header content included in the previous messages received from the sender.
 19. The system of claim 16, wherein analyzing the electronic message based at least in part on the previously observed communications between the sender and the intended recipient to determine the security risk of the electronic message for the intended recipient includes determining a plurality of risk component scores and combining the plurality of risk component scores to determine an overall risk score of the security risk of the electronic message for the intended recipient.
 20. A computer program product, the computer program product being embodied in a non-transitory computer readable storage medium and comprising computer instructions executable by a hardware processor for detecting security risk based on by performing: receiving information about an electronic message that is from a sender for an intended recipient; determining whether the sender of the electronic message has an established relationship with the intended recipient, wherein the established relationship is based at least in part on previous email messages between the sender and the intended recipient; in response to the determination that the sender of the electronic message has the established relationship with the intended recipient, using a processor to analyze the electronic message based at least in part on previously observed communications between the sender and the intended recipient to determine a security risk of the electronic message for the intended recipient using a statically analyzed result identifying a likelihood of existence of a cluster of two or more email header items by tracking and matching combinations of corresponding email header items from email headers of previous email messages sent from the sender, wherein the cluster of the combination of two or more of the email header items include a mail user agent (MUA) metadata item, a time zone, an IP address, X-header metadata information, or an identification of a supported character set; and based on the determined security risk of the electronic message, performing a security action, if applicable. 